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Business Segments (Details 4) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Segment Reporting Information [Line Items]    
Total assets $ 305,452us-gaap_Assets $ 280,156us-gaap_Assets
United States    
Segment Reporting Information [Line Items]    
Total assets 183,623us-gaap_Assets
/ us-gaap_StatementGeographicalAxis
= country_US		 194,285us-gaap_Assets
/ us-gaap_StatementGeographicalAxis
= country_US
United Kingdom    
Segment Reporting Information [Line Items]    
Total assets 68,285us-gaap_Assets
/ us-gaap_StatementGeographicalAxis
= country_GB		 56,481us-gaap_Assets
/ us-gaap_StatementGeographicalAxis
= country_GB
Other Countries    
Segment Reporting Information [Line Items]    
Total assets $ 53,544us-gaap_Assets
/ us-gaap_StatementGeographicalAxis
= gpx_OtherCountriesMember		 $ 29,390us-gaap_Assets
/ us-gaap_StatementGeographicalAxis
= gpx_OtherCountriesMember