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Exit Activity Costs and Asset Impairments (Summary Of Exit Activity Costs And Asset Impairments by Segment) (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
Corporate      
Restructuring Cost and Reserve [Line Items]      
Total exit activity costs and asset impairments 0us-gaap_BusinessExitCosts1
/ us-gaap_ConsolidationItemsAxis
= us-gaap_CorporateNonSegmentMember
0us-gaap_BusinessExitCosts1
/ us-gaap_ConsolidationItemsAxis
= us-gaap_CorporateNonSegmentMember
140us-gaap_BusinessExitCosts1
/ us-gaap_ConsolidationItemsAxis
= us-gaap_CorporateNonSegmentMember
Residential Products | Operating Segments      
Restructuring Cost and Reserve [Line Items]      
Total exit activity costs and asset impairments 752us-gaap_BusinessExitCosts1
/ us-gaap_ConsolidationItemsAxis
= us-gaap_OperatingSegmentsMember
/ us-gaap_StatementBusinessSegmentsAxis
= rock_ResidentialProductsMember
2,521us-gaap_BusinessExitCosts1
/ us-gaap_ConsolidationItemsAxis
= us-gaap_OperatingSegmentsMember
/ us-gaap_StatementBusinessSegmentsAxis
= rock_ResidentialProductsMember
2,457us-gaap_BusinessExitCosts1
/ us-gaap_ConsolidationItemsAxis
= us-gaap_OperatingSegmentsMember
/ us-gaap_StatementBusinessSegmentsAxis
= rock_ResidentialProductsMember
Industrial and Infrastructure Products | Operating Segments      
Restructuring Cost and Reserve [Line Items]      
Total exit activity costs and asset impairments $ 919us-gaap_BusinessExitCosts1
/ us-gaap_ConsolidationItemsAxis
= us-gaap_OperatingSegmentsMember
/ us-gaap_StatementBusinessSegmentsAxis
= rock_IndustrialAndInfrastructureProductsMember
$ 113us-gaap_BusinessExitCosts1
/ us-gaap_ConsolidationItemsAxis
= us-gaap_OperatingSegmentsMember
/ us-gaap_StatementBusinessSegmentsAxis
= rock_IndustrialAndInfrastructureProductsMember
$ 1,407us-gaap_BusinessExitCosts1
/ us-gaap_ConsolidationItemsAxis
= us-gaap_OperatingSegmentsMember
/ us-gaap_StatementBusinessSegmentsAxis
= rock_IndustrialAndInfrastructureProductsMember