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v3.19.3

Employee Benefits - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2019 | Sep. 30, 2018
401(k) Retirement Plan |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |
Company's share of discretionary matching contributions | $ 0.9 | $ 0.8

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_PlanNameAxis=geos_FourZeroOneKRetirementPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |