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Debt (Schedule of Long-Term Debt) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Jan. 31, 2013
Debt Instrument [Line Items]
Total debt	$ 213,600us-gaap_LongTermDebt	$ 214,007us-gaap_LongTermDebt
Less current maturities	400us-gaap_LongTermDebtCurrent	409us-gaap_LongTermDebtCurrent
Total long-term debt	213,200us-gaap_LongTermDebtNoncurrent	213,598us-gaap_LongTermDebtNoncurrent
Senior Subordinated 6.25% Notes
Debt Instrument [Line Items]
Total debt	210,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = rock_SeniorSubordinatedSixPointTwoFivePercentNotesDueFebruaryOneTwoThousandTwentyOneMember	210,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = rock_SeniorSubordinatedSixPointTwoFivePercentNotesDueFebruaryOneTwoThousandTwentyOneMember
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	6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = rock_SeniorSubordinatedSixPointTwoFivePercentNotesDueFebruaryOneTwoThousandTwentyOneMember
Other debt
Debt Instrument [Line Items]
Total debt	$ 3,600us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = rock_OtherDebtMember	$ 4,007us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = rock_OtherDebtMember

X

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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