Quarterly report pursuant to Section 13 or 15(d)

Subsequent Events (Details)

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 $ 615,000us-gaap_ProceedsFromIssuanceOfCommonStock                    
Fixed Price Swap, October through December 2015 [Member]                      
Subsequent Event [Line Items]                      
Daily Volume (in MMBtu per day)           262,500invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= gpor_FixedPriceSwapOctoberthroughDecember2015Member
         
Weighted Average Price (in usd per MMBtu)           3.96us-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   301,300,000us-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     501,900,000us-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)                   1,500invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= gpor_FixedPriceSwapJune2015throughJune2016Member
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
 
Weighted Average Price (in usd per MMBtu)                   62.47us-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)                   20,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= gpor_FixedPriceSwapOctoberthroughDecember2015Member
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
 
Weighted Average Price (in usd per MMBtu)                   3.26us-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)                   40,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= gpor_FixedPriceSwapJanuary2016throughDecember2016Member
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
 
Weighted Average Price (in usd per MMBtu)                   3.32us-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)                   20,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= gpor_FixedPriceSwapJanuary2017throughDecember2018Member
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
 
Weighted Average Price (in usd per MMBtu)                   3.26us-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)                   20,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= gpor_SwaptionContractJanuary2016throughDecember2016MemberMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
 
Weighted Average Price (in usd per MMBtu)                   3.38us-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       460,000gpor_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)       40,000gpor_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         350,000,000us-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         $ 343,600,000us-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)     10,925,000us-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)     1,425,000us-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                     24,000us-gaap_GasAndOilAreaUndevelopedNet
/ us-gaap_BusinessAcquisitionAxis
= gpor_PalomaPartnersIIILLCMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember