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Company's Derivatives on Consolidated Financial Statements of Operations (Details) (Foreign Currency Forward Contract, Other Income (Expense), USD $)
In Thousands, unless otherwise specified
3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Foreign Currency Forward Contract | Other Income (Expense)
   
Derivative Instruments Gain Loss [Line Items]    
Amount of (Loss) Gain Recognized in Income $ 925us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_IncomeStatementLocationAxis
= geos_OtherIncomeExpenseMember
$ (862)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_IncomeStatementLocationAxis
= geos_OtherIncomeExpenseMember