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Income Taxes (Narrative) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Operating Loss Carryforwards [Line Items] |  |  |
Alternative minimum tax credits | $ 24,053,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax | $ 7,968,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax | $ 1,643,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax
Foreign tax credit carryforwards | 2,074,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign | 2,074,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign | 2,074,000 us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign
Valuation allowance of deferred tax assets | 3,145,000 us-gaap_DeferredTaxAssetsValuationAllowance | 4,743,000 us-gaap_DeferredTaxAssetsValuationAllowance | 4,629,000 us-gaap_DeferredTaxAssetsValuationAllowance
Change in deferred tax asset valuation | 0 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance |  | (9,792,000)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Federal and state income taxes payable | 17,700,000 us-gaap_TaxesPayableCurrentAndNoncurrent | 11,000,000 us-gaap_TaxesPayableCurrentAndNoncurrent |
Diamondback [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Taxable gain on contribution | 203,300,000 gpor_TaxableGainOnContributionOfOilAndGasProperties / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = gpor_DiamondbackMember |  | 61,900,000 gpor_TaxableGainOnContributionOfOilAndGasProperties / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = gpor_DiamondbackMember
Change in deferred tax asset valuation | 79,400,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = gpor_DiamondbackMember |  | 0 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = gpor_DiamondbackMember
Deferred tax expense |  | 35,700,000 us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = gpor_DiamondbackMember | 9,800,000 us-gaap_DeferredIncomeTaxExpenseBenefit / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = gpor_DiamondbackMember
Deferred Other Tax Expense (Benefit) |  |  | 25,600,000 us-gaap_DeferredOtherTaxExpenseBenefit / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = gpor_DiamondbackMember
Gain from sale of equity investment |  | 120,000,000 us-gaap_GainLossOnSaleOfEquityInvestments / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = gpor_DiamondbackMember |
Current tax expense |  | 13,200,000 us-gaap_CurrentIncomeTaxExpenseBenefit / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = gpor_DiamondbackMember |
Federal [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating loss carryforward | 3,100,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_InternalRevenueServiceIRSMember |  |
Louisiana [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Net operating loss carryforward | 50,500,000 us-gaap_OperatingLossCarryforwards / us-gaap_IncomeTaxAuthorityAxis = us-gaap_StateAndLocalJurisdictionMember |  |
Blackhawk Midstream LLC [Member] |  |  |
Operating Loss Carryforwards [Line Items] |  |  |
Change in deferred tax asset valuation | 32,300,000 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gpor_BlackhawkMidstreamLlcMember |  |
Gain from sale of equity investment | $ 83,700,000 us-gaap_GainLossOnSaleOfEquityInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gpor_BlackhawkMidstreamLlcMember |  |

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- Definition Amount of current income tax expense (benefit) pertaining to taxable income (loss) from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I.7) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Current Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6509736 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 + Details
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- Definition Amount of other deferred income tax expense (benefit) pertaining to income (loss) from continuing operations. For example, but not limited to, acquisition-date income tax benefits or expenses recognized from changes in the acquirer's valuation allowance for its previously existing deferred tax assets resulting from a business combination and adjustments to beginning-of-year balance of a valuation allowance because of a change in circumstance causing a change in judgment about the realizability of the related deferred tax asset in future periods. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I.7) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 6 -Section I -Subsection 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Deferred Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6510177 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (g),(h) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible foreign tax credit carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 + Details
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- Definition The difference between the carrying value and the sale price of equity securities, not separately or otherwise categorized as trading or available-for-sale. This element includes investments in which the entity holds a small ownership stake (generally, less than 20% of the shares outstanding) and cannot exert significant influence. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(h)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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