Short-term Investments - Additional Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014
Short Term Investments [Abstract]
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss), Net of Tax	$ 28	$ 26

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