Consolidated Statement of Shareholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
Statement of Stockholders' Equity [Abstract]
Adjustment to pension benefit liability, taxes	$ 4	$ 5	$ 4	$ 10	$ 11	$ 10

Other Comprehensive Income Minimum Pension Liability Net Adjustment Tax 1

No definition available.

Name:	rock_OtherComprehensiveIncomeMinimumPensionLiabilityNetAdjustmentTax1
Namespace Prefix:	rock_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration