Revenue Recognition - Schedule of Net Contract Assets (Liabilities) (Detail) - USD ($) $ in Thousands	3 Months Ended
Jun. 30, 2026	Mar. 31, 2026
Revenue from Contract with Customer [Abstract]
Unbilled revenue (contract assets)	$ 65,884	$ 59,868
Customer deposits (contract liabilities)	(113,135)	(102,421)
Net contract (liabilities) assets	(47,251)	$ (42,553)
Unbilled revenue (contract assets)	6,016
Customer deposits (contract liabilities)	(10,714)
Net contract (liabilities) assets	(4,698)
Change due to revenue recognized (Contract assets)	34,328
Change due to revenue recognized (Contract Liabilities)	16,363
Change due to invoicing customers/ additional deposits (Contract Assets)	(28,312)
Change due to invoicing customers/ additional deposits (Contract Liabilities)	$ (27,077)

Change due to invoicing customers/ additional deposits (Contract Assets).

No definition available.

Name:	ghm_ChangeDueToInvoicingCustomersAdditionalDepositsContractAssets
Namespace Prefix:	ghm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Change due to invoicing customers/ additional deposits (Contract Liabilities).

+ References

No definition available.

+ Details

Name:	ghm_ChangeDueToInvoicingCustomersAdditionalDepositsContractLiabilities
Namespace Prefix:	ghm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Change due to revenue recognized contract assets

+ References

No definition available.

+ Details

Name:	ghm_ChangeDueToRevenueRecognizedContractAssets
Namespace Prefix:	ghm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Change due to revenue recognized Contract Liabilities

+ References

No definition available.

+ Details

Name:	ghm_ChangeDueToRevenueRecognizedContractLiabilities
Namespace Prefix:	ghm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The change in the balance of the contract with customer asset.

+ References

No definition available.

+ Details

Name:	ghm_ChangeInContractWithCustomerAsset
Namespace Prefix:	ghm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The change in the net balance of the contract with customer asset and contract with customer liability.

+ References

No definition available.

+ Details

Name:	ghm_ChangeInContractWithCustomerAssetLiabilityNet
Namespace Prefix:	ghm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The change in the balance of the contract with customer liability.

+ References

No definition available.

+ Details

Name:	ghm_ChangeInContractWithCustomerLiability
Namespace Prefix:	ghm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The net balance of current contract with customer assets and current contract with customer liabilities.

+ References

No definition available.

+ Details

Name:	ghm_ContractWithCustomerNetAssetLiabilityCurrent
Namespace Prefix:	ghm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-11

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

+ Details

Name:	us-gaap_ContractWithCustomerAssetNetCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 8
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483467/210-10-45-8

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-11

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-2

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration