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v2.4.1.9

Subsequent Events (Details) (USD $) | 3 Months Ended | 0 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2014 | Sep. 30, 2015 | Apr. 21, 2015 | Apr. 29, 2015 | Apr. 16, 2015 | Mar. 31, 2015 | Aug. 18, 2014 | Dec. 21, 2012 | Oct. 17, 2012 | Apr. 30, 2015 | Apr. 15, 2015 acre
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of common stock, net of offering costs and exercise of stock options | $ 615,000 us-gaap_ProceedsFromIssuanceOfCommonStock |  |  |  |  |  |  |  |  |  |
Fixed Price Swap, October through December 2015 [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Daily Volume (in MMBtu per day) |  |  |  |  |  | 262,500 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapOctoberthroughDecember2015Member |  |  |  |  |
Weighted Average Price (in usd per MMBtu) |  |  |  |  |  | 3.96 us-gaap_DerivativeSwapTypeAverageFixedPrice / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapOctoberthroughDecember2015Member |  |  |  |  |
Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  | 7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember | 7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember | 7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember |  |
Scenario, Forecast [Member] | Paloma Partners III, LLC [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Purchase price |  | 301,300,000 us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = gpor_PalomaPartnersIIILLCMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember |  |  |  |  |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of common stock, net of offering costs and exercise of stock options |  |  | 501,900,000 us-gaap_ProceedsFromIssuanceOfCommonStock / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |  |  |  |  |  |  |
Subsequent Event [Member] | Fixed Price Swap, June 2015 through June 2016 [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Daily Volume (in MMBtu per day) |  |  |  |  |  |  |  |  |  | 1,500 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapJune2015throughJune2016Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Weighted Average Price (in usd per MMBtu) |  |  |  |  |  |  |  |  |  | 62.47 us-gaap_DerivativeSwapTypeAverageFixedPrice / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapJune2015throughJune2016Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Subsequent Event [Member] | Fixed Price Swap, October through December 2015 [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Daily Volume (in MMBtu per day) |  |  |  |  |  |  |  |  |  | 20,000 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapOctoberthroughDecember2015Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Weighted Average Price (in usd per MMBtu) |  |  |  |  |  |  |  |  |  | 3.26 us-gaap_DerivativeSwapTypeAverageFixedPrice / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapOctoberthroughDecember2015Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Subsequent Event [Member] | Fixed Price Swap, January 2016 through December 2016 [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Daily Volume (in MMBtu per day) |  |  |  |  |  |  |  |  |  | 40,000 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapJanuary2016throughDecember2016Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Weighted Average Price (in usd per MMBtu) |  |  |  |  |  |  |  |  |  | 3.32 us-gaap_DerivativeSwapTypeAverageFixedPrice / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapJanuary2016throughDecember2016Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Subsequent Event [Member] | Fixed Price Swap, January 2017 through December 2018 [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Daily Volume (in MMBtu per day) |  |  |  |  |  |  |  |  |  | 20,000 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapJanuary2017throughDecember2018Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Weighted Average Price (in usd per MMBtu) |  |  |  |  |  |  |  |  |  | 3.26 us-gaap_DerivativeSwapTypeAverageFixedPrice / us-gaap_DerivativeByNatureAxis = gpor_FixedPriceSwapJanuary2017throughDecember2018Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Subsequent Event [Member] | Swaption Contract, January 2016 through December 2016 [Member] [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Daily Volume (in MMBtu per day) |  |  |  |  |  |  |  |  |  | 20,000 invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = gpor_SwaptionContractJanuary2016throughDecember2016MemberMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Weighted Average Price (in usd per MMBtu) |  |  |  |  |  |  |  |  |  | 3.38 us-gaap_DerivativeSwapTypeAverageFixedPrice / us-gaap_DerivativeByNatureAxis = gpor_SwaptionContractJanuary2016throughDecember2016MemberMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Subsequent Event [Member] | Chief Executive Officer [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Employment agreement, annual salary |  |  |  | 460,000 gpor_EmploymentAgreementAnnualCompensation / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefExecutiveOfficerMember |  |  |  |  |  |  |
Subsequent Event [Member] | Restricted Stock [Member] | Chief Executive Officer [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Employment agreement, annual grant (in shares) |  |  |  | 40,000 gpor_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsEmploymentAgreementAnnualGrantofShares / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefExecutiveOfficerMember |  |  |  |  |  |  |
Employment agreement, percent of salary awarded in shares |  |  |  | 500.00%gpor_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsEmploymentAgreementMaximumAnnalSalaryGrantedinOptionsPercent / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember / us-gaap_TitleOfIndividualAxis = us-gaap_ChiefExecutiveOfficerMember |  |  |  |  |  |  |
Subsequent Event [Member] | Senior Notes [Member] | Notes Due 2023 [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, amount |  |  |  |  | 350,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = gpor_NotesDue2023Member / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |  |  |  |  |
Stated interest rate |  |  |  |  | 6.625%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = gpor_NotesDue2023Member / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |  |  |  |  |
Proceeds from debt, net of issuance costs |  |  |  |  | $ 343,600,000 us-gaap_ProceedsFromDebtNetOfIssuanceCosts / us-gaap_DebtInstrumentAxis = gpor_NotesDue2023Member / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |  |  |  |  |
Subsequent Event [Member] | Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock issued in underwritten public offering (in shares) |  |  | 10,925,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |  |  |  |  |  |  |
Subsequent Event [Member] | Common Stock [Member] | Over-Allotment Option [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Stock issued in underwritten public offering (in shares) |  |  | 1,425,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementEquityComponentsAxis = us-gaap_CommonStockMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember / us-gaap_SubsidiarySaleOfStockAxis = us-gaap_OverAllotmentOptionMember |  |  |  |  |  |  |  |
Subsequent Event [Member] | Paloma Partners III, LLC [Member] |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |
Gas and oil acreage, undeveloped, net |  |  |  |  |  |  |  |  |  |  | 24,000 us-gaap_GasAndOilAreaUndevelopedNet / us-gaap_BusinessAcquisitionAxis = gpor_PalomaPartnersIIILLCMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Employment Agreement, Annual Grant of Shares + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Employment Agreement, Maximum Annal Salary Granted in Options, Percent + References No definition available. + Details
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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 30 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477 + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Fractional interest owned based on working interests or other economic arrangements of lease area on which wells have not been drilled or completed to a point that would permit the production of commercial quantities of oil and gas regardless of whether or not such area contains proved reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-K (SK) -Number 229 -Section 1208 -Paragraph (b) + Details
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