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Related Party Transactions - Additional Information (Detail) - Marcar [Member] - USD ($) |  | 12 Months Ended
--- | --- | ---
Oct. 05, 2017 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Related Party Transaction [Line Items] |  |  |  |
Lease payments to Marcar |  | $ 125,000 | $ 147,000 | $ 136,000
Subsequent Event [Member] | Leased Vehicles 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
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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
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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_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
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: |