The GS was Citroen's optimistic look at the future -- they shoved all kinds of crazy technology, like a self leveling suspension derived from the DS, semi-automatic gearbox, double-wishbone front/rear suspension, and 4 wheel disc brakes into what was supposed to be a simple family car.
From the three photos of this car, it is impossible to tell if this is the Berline (saloon) or fastback version -- in fact, the seller includes a strange picture of the underside of the car with some CRT monitor in the background, undoubtedly getting some advanced diagnosis from a Citroen expert for some problem...or just an oil change. The seller later identifies himself as zombiemotors, a Land Rover importer in Oregon who claims to be able to import and get a clean title for all sorts of oddballs. Probably a great guy to know!
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Image credit: citroenet.org.uk