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Acquisitions (Acquisition Related Costs) (Details) (September 2013 Acquisition, USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Business Acquisition [Line Items]    
Total acquisition related costs $ (228)us-gaap_BusinessCombinationAcquisitionRelatedCosts $ 208us-gaap_BusinessCombinationAcquisitionRelatedCosts
Selling, general and administrative costs    
Business Acquisition [Line Items]    
Total acquisition related costs (228)us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SellingGeneralAndAdministrativeExpensesMember		 2us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SellingGeneralAndAdministrativeExpensesMember
Cost of sales    
Business Acquisition [Line Items]    
Total acquisition related costs $ 0us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember		 $ 206us-gaap_BusinessCombinationAcquisitionRelatedCosts
/ us-gaap_BusinessAcquisitionAxis
= rock_September2013AcquisitionMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember