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Related Party Transactions - Additional Information (Detail) - Marcar [Member] - USD ($)		12 Months Ended
	Oct. 05, 2017	Sep. 30, 2018	Sep. 30, 2017
Related Party Transaction [Line Items]
Lease payments to Marcar		$ 0	$ 125,000
Leased vehicles and forklifts under contract [Member]
Related Party Transaction [Line Items]
Purchase of leased vehicles under contract	$ 320,000

X

- Definition

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

No definition available.

+ Details

Name:	us-gaap_LeaseAndRentalExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

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

X

- Definition

Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

No definition available.

+ Details

Name:	us-gaap_RelatedPartyTransactionPurchasesFromRelatedParty
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=genc_MarcarMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=genc_LeasedVehiclesUnderContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: