Short-term Investments - Additional Information (Details) (USD $)	6 Months Ended	12 Months Ended
Mar. 31, 2015	Sep. 30, 2014
Short Term Investments [Abstract]
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax	$ 5,000	$ 26,000

Amount after tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale

No definition available.

Name:	geos_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossNetOfTax
Namespace Prefix:	geos_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration