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Debt (Details Textual) (USD $)
3 Months Ended 0 Months Ended
Mar. 31, 2015
Sep. 02, 2014
Short-term Debt [Line Items]    
Borrowing capacity   65,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Long-term Debt, Gross   40,000,000us-gaap_DebtInstrumentCarryingAmount
Debt Instrument, Periodic Payment, Principal 1,100,000us-gaap_DebtInstrumentPeriodicPaymentPrincipal  
Weighted average interest rate 2.20%us-gaap_LineOfCreditFacilityInterestRateDuringPeriod  
Maximum [Member]    
Short-term Debt [Line Items]    
Line of Credit Facility, Interest Rate Description   LIBOR market index rate plus 2.50%
Minimum [Member]    
Short-term Debt [Line Items]    
Line of Credit Facility, Interest Rate Description LIBOR market index rate plus 1.25%  
Financing and Security Agreement [Member]    
Short-term Debt [Line Items]    
Borrowing outstanding 21,500,000us-gaap_LineOfCreditFacilityAmountOutstanding
/ us-gaap_LineOfCreditFacilityAxis
= gpx_FinancingAndSecurityAgreementMember
 
Available borrowings 42,600,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_LineOfCreditFacilityAxis
= gpx_FinancingAndSecurityAgreementMember
 
Financing and Security Agreement [Member] | LIBOR [Member] | Maximum [Member]    
Short-term Debt [Line Items]    
Interest rate spread 2.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LineOfCreditFacilityAxis
= gpx_FinancingAndSecurityAgreementMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
 
Financing and Security Agreement [Member] | LIBOR [Member] | Minimum [Member]    
Short-term Debt [Line Items]    
Interest rate spread 1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LineOfCreditFacilityAxis
= gpx_FinancingAndSecurityAgreementMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
 
Fourth Amended and Restated Financing and Security Agreement [Member] | Term Loan [Member]    
Short-term Debt [Line Items]    
Long-term Debt $ 34,400,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= gpx_FourthAmendedandRestatedFinancingandSecurityAgreementMember
/ us-gaap_LongtermDebtTypeAxis
= gpx_TermLoanMember