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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
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Restructuring Cost and Reserve [Line Items] |  |  |
Total exit activity costs and asset impairments | $ 1,671 us-gaap_BusinessExitCosts1 | $ 2,634 us-gaap_BusinessExitCosts1 | $ 4,004 us-gaap_BusinessExitCosts1
Cost of sales |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |
Total exit activity costs and asset impairments | 843 us-gaap_BusinessExitCosts1 / us-gaap_RestructuringCostAndReserveAxis = us-gaap_CostOfSalesMember | 2,519 us-gaap_BusinessExitCosts1 / us-gaap_RestructuringCostAndReserveAxis = us-gaap_CostOfSalesMember | 3,741 us-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 | $ 828 us-gaap_BusinessExitCosts1 / us-gaap_RestructuringCostAndReserveAxis = rock_SellingGeneralAndAdministrativeExpenseMember | $ 115 us-gaap_BusinessExitCosts1 / us-gaap_RestructuringCostAndReserveAxis = rock_SellingGeneralAndAdministrativeExpenseMember | $ 263 us-gaap_BusinessExitCosts1 / us-gaap_RestructuringCostAndReserveAxis = rock_SellingGeneralAndAdministrativeExpenseMember

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- Definition Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Includes, but is not limited to, one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, and relocating employees, and termination benefits associated with an ongoing benefit arrangement. Excludes expenses associated with special or contractual termination benefits, a discontinued operation or an asset retirement obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB TOPIC 5.P.4(b)) -URI http://asc.fasb.org/extlink&oid=27011515&loc=d3e140904-122747 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 420 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.P.3) -URI http://asc.fasb.org/extlink&oid=27011515&loc=d3e140864-122747 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 5 -Section P -Subsection 3, 4 + Details
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