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Property and Equipment - Additional Information (Detail) - USD ($) | 12 Months Ended
--- | ---
Sep. 30, 2017 | Sep. 30, 2016
Property, Plant and Equipment [Abstract] |  |
Property and equipment (fully depreciated assets) | $ 10,645,000 | $ 8,777,000

X
- Definition Property and equipment (fully depreciated assets). + References No definition available. + Details
Name: | genc_PropertyAndEquipmentFullyDepreciatedAssets
Namespace Prefix: | genc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration