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Note 3 - Revenue (Details Textual) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018
Revenue, Remaining Performance Obligation, Amount Greater than What Has Been Recorded Prior to Adoption of New Standard	$ 0.5		$ 0.5
Revenue, Remaining Performance Obligation, Amount	$ 128.9		$ 128.9
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year		1 year
Contract with Customer, Liability, Revenue Recognized			$ 3.3
Foreign Countries [Member]
Percentage Revenue By Location	36.00%	35.00%

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 greater than what has been recorded prior to adoption of new standard as transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

No definition available.

+ Details

Name:	grc_RevenueRemainingPerformanceObligationAmountGreaterThanWhatHasBeenRecordedPriorToAdoptionOfNewStandard
Namespace Prefix:	grc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: