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Note 9 - Business Segment Information (Details Textual) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Number of Reportable Segments | 1 |
Number of Countries in which Entity Operates | 150 |
UNITED STATES |  |
Long Lived Assets, Percent | 89.00% | 92.00%

X
- Definition The percentage of long lived assets. + References No definition available. + Details
Name: | grc_LongLivedAssetsPercent
Namespace Prefix: | grc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The number of countries in which the entity operates as of balance sheet date. + References No definition available. + Details
Name: | us-gaap_NumberOfCountriesInWhichEntityOperates
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementGeographicalAxis=country_US
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |