Derivative Financial Instruments - Additional Information (Details) (USD $) In Millions, unless otherwise specified		12 Months Ended
Sep. 30, 2014	Sep. 30, 2014 Foreign Currency Forward Contract	Sep. 30, 2014 Canadian Subsidiary	Sep. 30, 2013 Canadian Subsidiary
Derivative Financial Instruments [Line Items]
Denominated intercompany accounts payable			$ 26.6	$ 28.3
Foreign currency forward contract to hedge		30.0
Accrued unrealized foreign exchange gain	$ 0.8

Foreign currency transaction gain loss unrealized accrued.

No definition available.

Name:	geos_ForeignCurrencyTransactionGainLossUnrealizedAccrued
Namespace Prefix:	geos_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant