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Note 1 - Summary of Significant Accounting Policies - Amortization of Intangible Assets Lives (Details) | 12 Months Ended
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Dec. 31, 2017
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: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_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: |