Developed by Marmon-Herrington and based on the lengthened chassis of the GP, the Seep as it was soon called proved to be too heavy on land and too small on the water, at least in western eyes, and therefore saw its only serious western use at the Sicily landing in July, 1943, at which time its production had already ceased.

The Russians however, facing numerous wide rivers to be crossed and as many bridgeheads to be gained on their way to Berlin, found the GPA's abilities so useful that they developed an almost exact copy after the end of the war.