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Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets | $ 78,391 us-gaap_DerivativeAssetsCurrent | $ 324 us-gaap_DerivativeAssetsCurrent |
Asset retirement obligation capitalized | 9,295 us-gaap_AssetRetirementObligationLiabilitiesIncurred | 3,556 us-gaap_AssetRetirementObligationLiabilitiesIncurred | 2,195 us-gaap_AssetRetirementObligationLiabilitiesIncurred
Level 1 [Member] | Diamondback Energy LLC [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity investment in Diamondback |  | 178,708 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gpor_DiamondbackEnergyLlcMember |
Level 1 [Member] | Fixed Price Swaps [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets | 0 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = gpor_FixedPriceSwapsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 0 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = gpor_FixedPriceSwapsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Liabilities |  | 0 us-gaap_DerivativeLiabilities / us-gaap_DerivativeInstrumentRiskAxis = gpor_FixedPriceSwapsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Level 2 [Member] | Diamondback Energy LLC [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity investment in Diamondback |  | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gpor_DiamondbackEnergyLlcMember |
Level 2 [Member] | Fixed Price Swaps [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets | 102,839 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = gpor_FixedPriceSwapsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 845 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = gpor_FixedPriceSwapsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Liabilities |  | 23,646 us-gaap_DerivativeLiabilities / us-gaap_DerivativeInstrumentRiskAxis = gpor_FixedPriceSwapsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Asset retirement obligation capitalized | 9,300 us-gaap_AssetRetirementObligationLiabilitiesIncurred / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |  |
Level 3 [Member] | Diamondback Energy LLC [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity investment in Diamondback |  | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = gpor_DiamondbackEnergyLlcMember |
Level 3 [Member] | Fixed Price Swaps [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets | 0 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = gpor_FixedPriceSwapsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 0 us-gaap_DerivativeAssetsCurrent / us-gaap_DerivativeInstrumentRiskAxis = gpor_FixedPriceSwapsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Liabilities |  | $ 0 us-gaap_DerivativeLiabilities / us-gaap_DerivativeInstrumentRiskAxis = gpor_FixedPriceSwapsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |

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