Annual report pursuant to Section 13 and 15(d)

Stock-Based Compensation (Summary Of Stock Option Plans By Exercise Price) (Details)

v2.4.1.9
Stock-Based Compensation (Summary Of Stock Option Plans By Exercise Price) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Number Outstanding (shares) 5,000us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
Number Exercisable (shares) 5,000us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions
Exercise Price $9.07 [Member]  
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Exercise Price (usd per share) $ 9.07gpor_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsExercisePrice
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= gpor_ExercisePriceNinePointZeroSevenMember
Number Outstanding (shares) 5,000us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= gpor_ExercisePriceNinePointZeroSevenMember
Weighted Average Remaining Life (in years) 0 years 8 months 8 days
Number Exercisable (shares) 5,000us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions
/ us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis
= gpor_ExercisePriceNinePointZeroSevenMember