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Exit Activity Costs and Asset Impairments (Summary Of Exit Activity Costs And Asset Impairments Recorded in the Consolidated Statements of Operations) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Restructuring Cost and Reserve [Line Items]      
Total exit activity costs and asset impairments $ 1,671us-gaap_BusinessExitCosts1 $ 2,634us-gaap_BusinessExitCosts1 $ 4,004us-gaap_BusinessExitCosts1
Cost of sales      
Restructuring Cost and Reserve [Line Items]      
Total exit activity costs and asset impairments 843us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_CostOfSalesMember
2,519us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_CostOfSalesMember
3,741us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_CostOfSalesMember
Selling, general, and administrative expense      
Restructuring Cost and Reserve [Line Items]      
Total exit activity costs and asset impairments $ 828us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= rock_SellingGeneralAndAdministrativeExpenseMember
$ 115us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= rock_SellingGeneralAndAdministrativeExpenseMember
$ 263us-gaap_BusinessExitCosts1
/ us-gaap_RestructuringCostAndReserveAxis
= rock_SellingGeneralAndAdministrativeExpenseMember