The agency will undertake UK consumer and corporate campaigns, while working with its affiliate offices and other agencies in countries on the Continent.
Garmin claims to be the market leader in the US, and the appointment of GCI comes as the company seeks to expand in the growing European market. It plans to launch several new products in the region soon.
Other agencies understood to have pitched include Firefly Communications, Band & Brown Communications and Weber Shandwick.
European sales of sat-nav kit in the second quarter of 2006 were double that for the same period last year, according to research firm Canalys.
But some systems are attracting criticism for apparently providing bungled directions. Last month the BBC reported on pensioners trapped in their coach for hours in Gloucestershire.