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Note 3 - Revenue 1 (Details Textual) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Contract with Customer, Liability, Revenue Recognized | $ 4.5 | $ 4.4 |
Non-US [Member] |  |  |
Percentage Revenue By Location | 29.00% | 31.00% | 34.00%

X
- Definition The percentage of revenues by location. + References No definition available. + Details
Name: | grc_PercentageRevenueByLocation
Namespace Prefix: | grc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130549-203045 + Details
Name: | us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | srt_StatementGeographicalAxis=us-gaap_NonUsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |