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Derivative Financial Instruments - Additional Information (Details) (USD $)
In Millions, unless otherwise specified
 6 Months Ended
Mar. 31, 2015
Sep. 30, 2014
Canadian Dollar Forward Contract    
Derivative Financial Instruments [Line Items]    
Foreign currency forward contract to hedge $ 28.0us-gaap_DerivativeCostOfHedge
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember		  
Accrued unrealized foreign exchange gain 0.3geos_ForeignCurrencyTransactionGainLossUnrealizedAccrued
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember		  
Canadian Subsidiary    
Derivative Financial Instruments [Line Items]    
Denominated intercompany accounts payable $ 28.6geos_DenominatedIntercompanyAccountsPayable
/ dei_LegalEntityAxis
= geos_CanadianSubsidiaryMember		 $ 29.8geos_DenominatedIntercompanyAccountsPayable
/ dei_LegalEntityAxis
= geos_CanadianSubsidiaryMember