Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

+ Details

Name:	us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

+ Details

Name:	us-gaap_FinitelivedIntangibleAssetsAcquired1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase in assets, excluding financial assets and goodwill, lacking physical substance with an indefinite life, from an acquisition.

+ References

No definition available.

+ Details

Name:	us-gaap_IndefinitelivedIntangibleAssetsAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TechnologyEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_CustomerRelationshipsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=rock_BacklogMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=rock_TerraSmartLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TrademarksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=rock_SunfigCorporationArchitecturalMailboxesLLCDeltaSeparationsLLCAndThermoEnergySystemsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TrademarksMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_NoncompeteAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Acquisitions (Schedule of Acquired Intangible Assets) (Details) - USD ($) $ in Thousands		3 Months Ended
	Dec. 11, 2020	Mar. 31, 2021
Technology \| Minimum
Business Acquisition [Line Items]
Weighted-Average Amortization Period	5 years
Technology \| Maximum
Business Acquisition [Line Items]
Weighted-Average Amortization Period	15 years
Customer relationships \| Minimum
Business Acquisition [Line Items]
Weighted-Average Amortization Period	11 years
Customer relationships \| Maximum
Business Acquisition [Line Items]
Weighted-Average Amortization Period	13 years
Backlog
Business Acquisition [Line Items]
Weighted-Average Amortization Period	1 year
TerraSmart LLC
Business Acquisition [Line Items]
Fair Value		$ 64,150
TerraSmart LLC \| Trademarks
Business Acquisition [Line Items]
Fair Value		20,830
TerraSmart LLC \| Technology
Business Acquisition [Line Items]
Fair Value		$ 1,940
Weighted-Average Amortization Period		12 years
TerraSmart LLC \| Customer relationships
Business Acquisition [Line Items]
Fair Value		$ 35,110
Weighted-Average Amortization Period		12 years
TerraSmart LLC \| Backlog
Business Acquisition [Line Items]
Fair Value		$ 6,270
Sunfig Corporation, Architectural Mailboxes, LLC, Delta Separations, LLC, and Thermo Energy Systems Inc.
Business Acquisition [Line Items]
Fair Value	$ 38,066
Sunfig Corporation, Architectural Mailboxes, LLC, Delta Separations, LLC, and Thermo Energy Systems Inc. \| Trademarks
Business Acquisition [Line Items]
Fair Value	8,100
Sunfig Corporation, Architectural Mailboxes, LLC, Delta Separations, LLC, and Thermo Energy Systems Inc. \| Trademarks
Business Acquisition [Line Items]
Fair Value	$ 1,177
Weighted-Average Amortization Period	3 years
Sunfig Corporation, Architectural Mailboxes, LLC, Delta Separations, LLC, and Thermo Energy Systems Inc. \| Technology
Business Acquisition [Line Items]
Fair Value	$ 8,345
Sunfig Corporation, Architectural Mailboxes, LLC, Delta Separations, LLC, and Thermo Energy Systems Inc. \| Customer relationships
Business Acquisition [Line Items]
Fair Value	18,480
Sunfig Corporation, Architectural Mailboxes, LLC, Delta Separations, LLC, and Thermo Energy Systems Inc. \| Non-compete agreements
Business Acquisition [Line Items]
Fair Value	$ 1,036
Weighted-Average Amortization Period	5 years
Sunfig Corporation, Architectural Mailboxes, LLC, Delta Separations, LLC, and Thermo Energy Systems Inc. \| Backlog
Business Acquisition [Line Items]
Fair Value	$ 928