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Note 1 - Summary of Significant Accounting Policies - Amortization of Intangible Assets Lives (Details)	12 Months Ended
	Dec. 31, 2020
Technology and Drawings [Member] | Minimum [Member]
Intangible assets lives (Year)	13 years
Technology and Drawings [Member] | Maximum [Member]
Intangible assets lives (Year)	20 years
Customer Relationships [Member] | Minimum [Member]
Intangible assets lives (Year)	9 years
Customer Relationships [Member] | Maximum [Member]
Intangible assets lives (Year)	15 years
Other Intangible Assets [Member] | Minimum [Member]
Intangible assets lives (Year)	2 years
Other Intangible Assets [Member] | Maximum [Member]
Intangible assets lives (Year)	18 years

X

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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

X

- Details

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