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Equity-Based Compensation (Narrative) (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
1 Months Ended 12 Months Ended
Jun. 30, 2014
Jan. 31, 2014
Jan. 31, 2013
Jan. 31, 2012
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Typical vesting period, in years         4 years    
Number of shares available for issuance under the Plan         376,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant    
Options granted         0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Number of restricted stock units credited to participant accounts         647,371rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount 614,888rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount  
Market price of common stock, per share         16.26rock_MarketPriceOfCommonStockPerShare    
Requisite vesting age         60 years    
Intrinsic value of options exercisable         1,393us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1    
Weighted average remaining life of options exercisable         4 years    
Unrecognized compensation cost         4,300us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized    
Weighted average cost recognition period, in years         3 years 3 months 18 days    
2005 Equity Incentive Plan              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Maximum number of shares that may be issued under the Plan         3,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_StatementEquityComponentsAxis
= rock_TwoThousandFiveEquityIncentivePlanMember
   
Maximum number of shares that may be issued in connection with grants of incentive stock options and rights         900,000rock_MaximumNumberOfSharesIssuedInConnectionWithGrantsOfIncentiveStockOptionsAndRights
/ us-gaap_StatementEquityComponentsAxis
= rock_TwoThousandFiveEquityIncentivePlanMember
   
Performance Stock Units              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Percentage of target performance stock units earned           50.00%rock_EquityBasedCompensationPercentOfTargetedPerformanceStockUnitsEarned
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
58.30%rock_EquityBasedCompensationPercentOfTargetedPerformanceStockUnitsEarned
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
Targeted Performance Stock Units           237,000rock_TargetedPerformanceStockUnits
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
280,000rock_TargetedPerformanceStockUnits
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
Performance stock units outstanding           237,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
280,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
Shares awarded 19,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
212,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
304,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
295,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
     
Performance stock unit grant-date fair value $ 319rock_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedGrantDateFairValue
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
$ 3,914rock_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedGrantDateFairValue
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
$ 4,123rock_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedGrantDateFairValue
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
$ 4,152rock_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedGrantDateFairValue
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
     
Performance stock units earned           114,000rock_PerformanceStockUnitsEarned
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
163,200rock_PerformanceStockUnitsEarned
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember
Service period         5 years    
Closing price period for determination of cash conversion         90 days    
Management Stock Purchase Plan              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Performance stock units outstanding         224,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
   
Closing price period for determination of cash conversion         200 days    
Weighted average fair value of per unit awarded         15.68us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
15.97us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
 
Number of restricted stock units credited to participant accounts         119,105rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
132,037rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
253,587rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
Accrues interest         2.00%rock_PercentageOfAccruesInterest
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
   
Lump sum payment option one         1rock_SharebasedCompensationArrangementbySharebasedPaymentAwardLumpSumPaymentOptionOne
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
   
Lump sum payment option two         5rock_SharebasedCompensationArrangementbySharebasedPaymentAwardLumpSumPaymentOptionTwo
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
   
Lump sum payment option three         10rock_SharebasedCompensationArrangementbySharebasedPaymentAwardLumpSumPaymentOptionThree
/ us-gaap_StatementEquityComponentsAxis
= rock_ManagementStockPurchasePlanMember
   
Maximum | Performance Stock Units              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Performance stock unit vesting period         3 years    
Minimum | Performance Stock Units              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Performance stock unit vesting period         1 year    
Unvested Restricted Stock Units              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Number of restricted stock units credited to participant accounts         62,455rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount
/ us-gaap_AwardTypeAxis
= rock_UnvestedRestrictedStockUnitMember
47,268rock_NumberOfRestrictedStockUnitsCreditedToParticipantAccount
/ us-gaap_AwardTypeAxis
= rock_UnvestedRestrictedStockUnitMember