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Debt (Details Textual) (USD $)
12 Months Ended 0 Months Ended 3 Months Ended
Dec. 31, 2014
Sep. 02, 2014
Dec. 31, 2014
Dec. 31, 2013
Short-term Debt [Line Items]        
Long-term Debt $ 37,777,000us-gaap_LongTermDebt   $ 37,777,000us-gaap_LongTermDebt  
Short-term borrowings 20,799,000us-gaap_LineOfCreditFacilityAmountOutstanding   20,799,000us-gaap_LineOfCreditFacilityAmountOutstanding 407,000us-gaap_LineOfCreditFacilityAmountOutstanding
Revolving Credit Facility [Member]        
Short-term Debt [Line Items]        
Short-term borrowings 20,800,000us-gaap_LineOfCreditFacilityAmountOutstanding
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
  20,800,000us-gaap_LineOfCreditFacilityAmountOutstanding
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
 
Available borrowing capacity 43,300,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
  43,300,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
 
Interest rate during period (percent) 1.70%us-gaap_LineOfCreditFacilityInterestRateDuringPeriod
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
     
Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]        
Short-term Debt [Line Items]        
Spread on variable rate (percent)   2.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= gpx_FourthAmendedandRestatedFinancingandSecurityAgreementMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
   
Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]        
Short-term Debt [Line Items]        
Spread on variable rate (percent)   1.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= gpx_FourthAmendedandRestatedFinancingandSecurityAgreementMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
   
Credit Agreement [Member] | Term Loan [Member]        
Short-term Debt [Line Items]        
Long-term Debt, Gross   40,000,000us-gaap_DebtInstrumentCarryingAmount
/ us-gaap_DebtInstrumentAxis
= gpx_FourthAmendedandRestatedFinancingandSecurityAgreementMember
/ us-gaap_LongtermDebtTypeAxis
= gpx_TermLoanMember
   
Periodic principal payments     1,100,000us-gaap_DebtInstrumentPeriodicPaymentPrincipal
/ us-gaap_DebtInstrumentAxis
= gpx_FourthAmendedandRestatedFinancingandSecurityAgreementMember
/ us-gaap_LongtermDebtTypeAxis
= gpx_TermLoanMember
 
Long-term Debt 37,800,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= gpx_FourthAmendedandRestatedFinancingandSecurityAgreementMember
/ us-gaap_LongtermDebtTypeAxis
= gpx_TermLoanMember
  37,800,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= gpx_FourthAmendedandRestatedFinancingandSecurityAgreementMember
/ us-gaap_LongtermDebtTypeAxis
= gpx_TermLoanMember
 
Credit Agreement [Member] | Revolving Credit Facility [Member]        
Short-term Debt [Line Items]        
Maximum borrowing capacity   65,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_CreditFacilityAxis
= us-gaap_RevolvingCreditFacilityMember
/ us-gaap_DebtInstrumentAxis
= gpx_FourthAmendedandRestatedFinancingandSecurityAgreementMember