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Employee Retirement Plans (Schedule of Contributions Made by the Company to Each Multiemployer Plan) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions | $ 329 us-gaap_MultiemployerPlanPeriodContributions | $ 328 us-gaap_MultiemployerPlanPeriodContributions | $ 333 us-gaap_MultiemployerPlanPeriodContributions
National Integrated Group Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions | 233 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_NationalIntegratedGroupPensionPlanMember | 222 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_NationalIntegratedGroupPensionPlanMember | 202 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_NationalIntegratedGroupPensionPlanMember
Sheet Metal Workers National Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions | 61 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersNationalPensionPlanMember | 78 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersNationalPensionPlanMember | 98 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersNationalPensionPlanMember
Sheet Metal Workers Pension Plan Of Northern California [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions | $ 35 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersPensionPlanOfNorthernCaliforniaMember | $ 28 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersPensionPlanOfNorthernCaliforniaMember | $ 33 us-gaap_MultiemployerPlanPeriodContributions / us-gaap_MultiemployerPlanNameAxis = rock_SheetMetalWorkersPensionPlanOfNorthernCaliforniaMember

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- Definition Amount of contributions made to multiemployer plans by the employer. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450702-114947 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450691-114947 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 80 -Section 50 -Paragraph 5 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=35742057&loc=SL14450657-114947 + Details
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