<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.24.1.u1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>246</ContextCount>
  <ElementCount>260</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>74</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Statement - Condensed Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/CondensedConsolidatedBalanceSheets</Role>
      <ShortName>Condensed Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>0000003 - Statement - Condensed Consolidated Balance Sheets (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/CondensedConsolidatedBalanceSheetsParentheticals</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>0000004 - Statement - Condensed Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/CondensedConsolidatedStatementsofOperations</Role>
      <ShortName>Condensed Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>0000005 - Statement - Condensed Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/CondensedConsolidatedStatementsofComprehensiveIncome</Role>
      <ShortName>Condensed Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>0000006 - Statement - Condensed Consolidated Statements of Changes in Stockholders??? Equity (Deficit)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/CondensedConsolidatedStatementsofChangesinStockholdersEquityDeficit</Role>
      <ShortName>Condensed Consolidated Statements of Changes in Stockholders??? Equity (Deficit)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>0000007 - Statement - Condensed Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/CondensedConsolidatedStatementsofCashFlows</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>0000008 - Disclosure - Business and Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/BusinessandBasisofPresentation</Role>
      <ShortName>Business and Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>0000009 - Disclosure - Acquisitions and Divestiture</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/AcquisitionsandDivestiture</Role>
      <ShortName>Acquisitions and Divestiture</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>0000010 - Disclosure - Goodwill and Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/GoodwillandIntangibleAssets</Role>
      <ShortName>Goodwill and Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>0000011 - Disclosure - Revenue and Related Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/RevenueandRelatedMatters</Role>
      <ShortName>Revenue and Related Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>0000012 - Disclosure - Computation of Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/ComputationofEarningsPerShare</Role>
      <ShortName>Computation of Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>0000013 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/StockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>0000014 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>0000015 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/Debt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>0000016 - Disclosure - Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/Equity</Role>
      <ShortName>Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>0000017 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>0000018 - Disclosure - Derivatives and Hedging</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/DerivativesandHedging</Role>
      <ShortName>Derivatives and Hedging</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>0000019 - Disclosure - Fair Value Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/FairValueDisclosures</Role>
      <ShortName>Fair Value Disclosures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>0000020 - Disclosure - Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/Contingencies</Role>
      <ShortName>Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>0000021 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9954471 - Disclosure - Business and Basis of Presentation (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/BusinessandBasisofPresentationPolicies</Role>
      <ShortName>Business and Basis of Presentation (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9954472 - Disclosure - Business and Basis of Presentation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/BusinessandBasisofPresentationTables</Role>
      <ShortName>Business and Basis of Presentation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/BusinessandBasisofPresentation</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9954473 - Disclosure - Goodwill and Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/GoodwillandIntangibleAssetsTables</Role>
      <ShortName>Goodwill and Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/GoodwillandIntangibleAssets</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9954474 - Disclosure - Revenue and Related Matters (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/RevenueandRelatedMattersTables</Role>
      <ShortName>Revenue and Related Matters (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/RevenueandRelatedMatters</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9954475 - Disclosure - Computation of Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/ComputationofEarningsPerShareTables</Role>
      <ShortName>Computation of Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/ComputationofEarningsPerShare</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9954476 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/StockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/StockBasedCompensation</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9954477 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/SegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/SegmentInformation</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9954478 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/DebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/Debt</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9954479 - Disclosure - Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/EquityTables</Role>
      <ShortName>Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/Equity</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9954480 - Disclosure - Derivatives and Hedging (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/DerivativesandHedgingTables</Role>
      <ShortName>Derivatives and Hedging (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/DerivativesandHedging</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9954481 - Disclosure - Fair Value Disclosures (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/FairValueDisclosuresTables</Role>
      <ShortName>Fair Value Disclosures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/FairValueDisclosures</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9954482 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.gartner.com/role/Leases</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9954483 - Disclosure - Business and Basis of Presentation - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/BusinessandBasisofPresentationNarrativeDetails</Role>
      <ShortName>Business and Basis of Presentation - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9954484 - Disclosure - Business and Basis of Presentation - Cash and Cash Equivalents and Restricted Cash (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/BusinessandBasisofPresentationCashandCashEquivalentsandRestrictedCashDetails</Role>
      <ShortName>Business and Basis of Presentation - Cash and Cash Equivalents and Restricted Cash (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9954485 - Disclosure - Acquisition and Divestiture (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/AcquisitionandDivestitureDetails</Role>
      <ShortName>Acquisition and Divestiture (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9954486 - Disclosure - Goodwill and Intangible Assets - Changes to the Carrying Amount of Goodwill by Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/GoodwillandIntangibleAssetsChangestotheCarryingAmountofGoodwillbySegmentDetails</Role>
      <ShortName>Goodwill and Intangible Assets - Changes to the Carrying Amount of Goodwill by Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9954487 - Disclosure - Goodwill and Intangible Assets - Carrying Amounts of Amortizable Intangible Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/GoodwillandIntangibleAssetsCarryingAmountsofAmortizableIntangibleAssetsDetails</Role>
      <ShortName>Goodwill and Intangible Assets - Carrying Amounts of Amortizable Intangible Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9954488 - Disclosure - Goodwill and Intangible Assets - Estimated Future Amortization Expense By Year From Amortizable Intangibles (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/GoodwillandIntangibleAssetsEstimatedFutureAmortizationExpenseByYearFromAmortizableIntangiblesDetails</Role>
      <ShortName>Goodwill and Intangible Assets - Estimated Future Amortization Expense By Year From Amortizable Intangibles (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9954489 - Disclosure - Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/RevenueandRelatedMattersDisaggregationofRevenuebyGeographicAreaDetails</Role>
      <ShortName>Revenue and Related Matters - Disaggregation of Revenue by Geographic Area (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9954490 - Disclosure - Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/RevenueandRelatedMattersTimingOfRevenueRecognitionLiabilitiesperSegmentDetails</Role>
      <ShortName>Revenue and Related Matters - Timing Of Revenue Recognition Liabilities per Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9954491 - Disclosure - Revenue and Related Matters - Revenue Remaining Performance Obligation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/RevenueandRelatedMattersRevenueRemainingPerformanceObligationDetails</Role>
      <ShortName>Revenue and Related Matters - Revenue Remaining Performance Obligation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9954492 - Disclosure - Revenue and Related Matters - Schedule of Contract with Customer, Asset and Liability (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/RevenueandRelatedMattersScheduleofContractwithCustomerAssetandLiabilityDetails</Role>
      <ShortName>Revenue and Related Matters - Schedule of Contract with Customer, Asset and Liability (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9954493 - Disclosure - Computation of Earnings Per Share - Calculations Of Basic And Diluted (Loss) Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/ComputationofEarningsPerShareCalculationsOfBasicAndDilutedLossEarningsPerShareDetails</Role>
      <ShortName>Computation of Earnings Per Share - Calculations Of Basic And Diluted (Loss) Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9954494 - Disclosure - Computation of Earnings Per Share - Additional information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/ComputationofEarningsPerShareAdditionalinformationDetails</Role>
      <ShortName>Computation of Earnings Per Share - Additional information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9954495 - Disclosure - Stock-Based Compensation - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/StockBasedCompensationNarrativeDetails</Role>
      <ShortName>Stock-Based Compensation - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9954496 - Disclosure - Stock-Based Compensation - Stock-based Compensation Expense by Award Type and Expense Category (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/StockBasedCompensationStockbasedCompensationExpensebyAwardTypeandExpenseCategoryDetails</Role>
      <ShortName>Stock-Based Compensation - Stock-based Compensation Expense by Award Type and Expense Category (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9954497 - Disclosure - Segment Information - Information About Reportable Segments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/SegmentInformationInformationAboutReportableSegmentsDetails</Role>
      <ShortName>Segment Information - Information About Reportable Segments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9954498 - Disclosure - Segment Information - Reconciliation of Segment Gross Contribution to Net Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/SegmentInformationReconciliationofSegmentGrossContributiontoNetIncomeDetails</Role>
      <ShortName>Segment Information - Reconciliation of Segment Gross Contribution to Net Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9954499 - Disclosure - Debt - Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/DebtBorrowingsDetails</Role>
      <ShortName>Debt - Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9954500 - Disclosure - Debt - Footnote to Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/DebtFootnotetoBorrowingsDetails</Role>
      <ShortName>Debt - Footnote to Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9954501 - Disclosure - Debt - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/DebtNarrativeDetails</Role>
      <ShortName>Debt - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9954502 - Disclosure - Debt - Interest Rate Swaps (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/DebtInterestRateSwapsDetails</Role>
      <ShortName>Debt - Interest Rate Swaps (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9954503 - Disclosure - Equity - Share Repurchase Program and Share Repurchase Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/EquityShareRepurchaseProgramandShareRepurchaseActivityDetails</Role>
      <ShortName>Equity - Share Repurchase Program and Share Repurchase Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9954504 - Disclosure - Equity - Changes in AOCI/L by Component (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/EquityChangesinAOCILbyComponentDetails</Role>
      <ShortName>Equity - Changes in AOCI/L by Component (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9954505 - Disclosure - Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/IncomeTaxesDetails</Role>
      <ShortName>Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.gartner.com/role/IncomeTaxes</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9954506 - Disclosure - Derivatives and Hedging - Outstanding Derivatives Contracts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/DerivativesandHedgingOutstandingDerivativesContractsDetails</Role>
      <ShortName>Derivatives and Hedging - Outstanding Derivatives Contracts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9954507 - Disclosure - Derivatives and Hedging - Derivative Gains And Losses That Have Been Recognized In Condensed Consolidated Statements Of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/DerivativesandHedgingDerivativeGainsAndLossesThatHaveBeenRecognizedInCondensedConsolidatedStatementsOfOperationsDetails</Role>
      <ShortName>Derivatives and Hedging - Derivative Gains And Losses That Have Been Recognized In Condensed Consolidated Statements Of Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9954508 - Disclosure - Fair Value Disclosures - Assets And Liabilities Measured At Fair Value On Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/FairValueDisclosuresAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>Fair Value Disclosures - Assets And Liabilities Measured At Fair Value On Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9954509 - Disclosure - Fair Value Disclosures - Fair Value of Debt Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/FairValueDisclosuresFairValueofDebtInstrumentsDetails</Role>
      <ShortName>Fair Value Disclosures - Fair Value of Debt Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9954510 - Disclosure - Fair Value Disclosures - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/FairValueDisclosuresNarrativeDetails</Role>
      <ShortName>Fair Value Disclosures - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9954511 - Disclosure - Leases - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/LeasesNarrativeDetails</Role>
      <ShortName>Leases - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9954512 - Disclosure - Leases - Net Lease Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/LeasesNetLeaseCostsDetails</Role>
      <ShortName>Leases - Net Lease Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="it-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9954513 - Disclosure - Leases - Supplemental Balance Sheet Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.gartner.com/role/LeasesSupplementalBalanceSheetInformationDetails</Role>
      <ShortName>Leases - Supplemental Balance Sheet Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Warning">[ix-0514-Hidden-Fact-Eligible-For-Transform] WARN: 4 fact(s) appearing in ix:hidden were eligible for transformation: us-gaap:AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount, us-gaap:DebtInstrumentBasisSpreadOnVariableRate1, us-gaap:RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1 -  it-20240331.htm 4</Log>
    <Log type="Warning">[ix-0514-Hidden-Fact-Not-Referenced] WARN: 1 fact(s) appearing in ix:hidden were not referenced by any -sec-ix-hidden style property: us-gaap:RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1 -  it-20240331.htm 4</Log>
    <Log type="Warning">[DQC.US.0076.7653] The filer has reported a value for us-gaap:RevenueRemainingPerformanceObligation with a value of 800000000.000000 using the dimension member 2026-01-01 The filer has not reported a duration value for the corresponding elements of RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1.

The properties of this us-gaap:RevenueRemainingPerformanceObligation fact are:
Context: c-116
Unit: usd. - it-20240331.htm 4 - it-20240331.htm 4</Log>
  </Logs>
  <InputFiles>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="it-20240331.htm">it-20240331.htm</File>
    <File>it-20240331.xsd</File>
    <File>it-20240331_cal.xml</File>
    <File>it-20240331_def.xml</File>
    <File>it-20240331_lab.xml</File>
    <File>it-20240331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="733">http://fasb.org/us-gaap/2023</BaseTaxonomy>
    <BaseTaxonomy items="30">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
