Exit Activity Costs and Asset Impairments (Narrative) (Details) - facility	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Restructuring and Related Activities [Abstract]
Number of consolidated facilities to be closed	1	3

Restructuring and Related Cost, Number of Facilities to be Closed

No definition available.

Name:	rock_RestructuringandRelatedCostNumberofFacilitiestobeClosed
Namespace Prefix:	rock_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration