Equity-Based Compensation (Schedule of Number of Awards and Weighted Average Grant Date Fair Value) (Details) - $ / shares	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Performance stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of awards (in shares)	108,464	62,778
Weighted average grant date fair value (in USD per share)	$ 47.00	$ 87.84
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of awards (in shares)	58,958	33,187
Weighted average grant date fair value (in USD per share)	$ 47.00	$ 87.91
Minimum \| Performance stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants based on targeted thresholds, percent	0.00%
Maximum \| Performance stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants based on targeted thresholds, percent	200.00%
Return on Invested Capital \| 2018 Plan \| Performance stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period	1 year
Total Shareholder Return \| 2018 Plan \| Performance stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period	3 years

Share-based Compensation Arrangement by Share-based Payment Award, Award Performance Period

No definition available.

Name:	rock_SharebasedCompensationArrangementbySharebasedPaymentAwardAwardPerformancePeriod
Namespace Prefix:	rock_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other Than Options, Grants Based On Targeted Thresholds, Percent

+ References

No definition available.

+ Details

Name:	rock_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsBasedOnTargetedThresholdsPercent
Namespace Prefix:	rock_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_PerformanceSharesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
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_MeasurementInputTypeAxis=rock_SharebasedCompensationMeasurementReturnOnInvestedCapitalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=rock_A2018PlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=rock_SharebasedCompensationMeasurementTotalShareholderReturnMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: