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Fair Value Measurements (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 trail	Dec. 31, 2013 business	Jan. 31, 2013
Extinguishment of Debt [Line Items]
Number of completed acquisitions		1us-gaap_NumberOfBusinessesAcquired
Carrying value of outstanding debt	$ 213,600us-gaap_LongTermDebt	$ 214,007us-gaap_LongTermDebt
Triangular distribution, number of trails	500,000rock_DiscountedCashFlowAnalysisTriangularDistributionNumberOfTrails
Senior Subordinated 6.25% Notes
Extinguishment of Debt [Line Items]
Carrying value of outstanding 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

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Discounted cash flow analysis triangular distribution, number of trails.

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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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The number of businesses acquired by the entity during the period.

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