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Inventories - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2014
Inventory Disclosure [Abstract] |  |  |
Inventory obsolescence expense and inventory write-downs | $ 11.2 | $ 3.9 | $ 2.6

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- Definition Inventory obsolescence expense and inventory write-downs. + References No definition available. + Details
Name: | geos_InventoryObsolescenceExpenseAndInventoryWriteDowns
Namespace Prefix: | geos_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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