Quarterly report pursuant to Section 13 or 15(d)

Fair Value of Financial Instruments (Details)

v2.4.1.9
Fair Value of Financial Instruments (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Aug. 18, 2014
Dec. 21, 2012
Oct. 17, 2012
Debt Instrument [Line Items]        
Fair value of notes $ 615.8us-gaap_DebtInstrumentFairValue      
October Notes [Member]        
Debt Instrument [Line Items]        
Unamortized discount 2.8us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_DebtInstrumentAxis
= gpor_OctoberNotesMember
     
December Notes [Member]        
Debt Instrument [Line Items]        
Unamortized premium 0.4us-gaap_DebtInstrumentUnamortizedPremium
/ us-gaap_DebtInstrumentAxis
= gpor_DecemberNotesMember
     
August Notes [Member]        
Debt Instrument [Line Items]        
Unamortized premium 16.5us-gaap_DebtInstrumentUnamortizedPremium
/ us-gaap_DebtInstrumentAxis
= gpor_AugustNotesMember
     
Senior Notes [Member]        
Debt Instrument [Line Items]        
Carrying value of debt 614.1us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
     
Unamortized discount       3.7us-gaap_DebtInstrumentUnamortizedDiscount
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Unamortized premium   $ 18.0us-gaap_DebtInstrumentUnamortizedPremium
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
$ 0.5us-gaap_DebtInstrumentUnamortizedPremium
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember