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Gross Fair Value of all Derivative Instruments (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Sep. 30, 2018
--- | --- | ---
Foreign Currency Forward Contracts | Accrued Expenses and Other Current Liabilities |  |
Derivatives Fair Value [Line Items] |  |
Derivatives Liabilities | $ 7 | $ 270

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- Definition Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_BalanceSheetLocationAxis=geos_AccruedExpensesAndOtherCurrentLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |