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Stock-Based Compensation (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total stock-based compensation expense $ 612us-gaap_AllocatedShareBasedCompensationExpense $ 506us-gaap_AllocatedShareBasedCompensationExpense
Board of Directors stock grants [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total stock-based compensation expense 111us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= us-gaap_BoardOfDirectorsChairmanMember		 82us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_TitleOfIndividualAxis
= us-gaap_BoardOfDirectorsChairmanMember
Non-qualified stock options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total stock-based compensation expense 62us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= gpx_NonQualifiedStockOptionsMember		 132us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= gpx_NonQualifiedStockOptionsMember
Restricted stock units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total stock-based compensation expense $ 439us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember		 $ 292us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember