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Supplemental Financial Information (Narrative) (Details)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2013
Schedule Of Supplemental Financial Information [Line Items]    
Ownership percentage 100.00%rock_OwnershipPercentageOfSubsidiaryGuarantorByParentCompany  
Senior Subordinated 6.25% Notes    
Schedule Of Supplemental Financial Information [Line Items]    
Senior Subordinated Notes, interest rate 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= rock_SeniorSubordinatedSixPointTwoFivePercentNotesDueFebruaryOneTwoThousandTwentyOneMember
6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= rock_SeniorSubordinatedSixPointTwoFivePercentNotesDueFebruaryOneTwoThousandTwentyOneMember