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Revenue Recognition and Related Costs - Additional Information (Detail) - USD ($)	6 Months Ended
	Mar. 31, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Amortization period for incremental costs	1 year
Costs and estimated earnings in excess of billings	$ 1,629,000	$ 1,903,000
Contract assets collection period	1 year
Accounts receivable related to contracts with customers	$ 130,000	210,000
Current Liabilities [Member]
Disaggregation of Revenue [Line Items]
Customer deposits related to contracts with customers	$ 10,286,000	$ 5,244,000

X

- Definition

Contract with customer accounts receivable.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Costs And Estimated Earnings In Excess Of Billings On Uncompleted Contracts Current.

+ References

No definition available.

+ Details

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Namespace Prefix:	genc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Expected period for contract assets billing and collection.

+ References

No definition available.

+ Details

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Namespace Prefix:	genc_
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Balance Type:	na
Period Type:	duration

X

- Definition

Incremental costs amortization period.

+ References

No definition available.

+ Details

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Namespace Prefix:	genc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=123360276&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130549-203045

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=123360276&loc=SL49130532-203044

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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X

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