Over 12 thousand were made and they served with Allied forces in the many theatres of WW2, including the Pacific, Eastern front, and from D-day to the end.

It was considered too slow and heavy on land, and lacked sufficient seagoing abilities in open water, due mainly to its low freeboard.

The Soviet Union received one third of the total GPA production under Lend Lease, and were sufficiently satisfied with its ability to cross calmer inland waters, that they produced a copy, the GAZ-46.

After having commissioned Willys, Ford and Bantam to build the first 4,500 jeeps (1500 each) in March 1941, the US Motor Transport Board set up a project under the direction of the National Defense Research Committee (NDRC) to be designated "QMC-4 1/4 Ton Truck Light Amphibian".

Stephens' hull design looked like a miniature version of that of the DUKW, and just like it, the 'Seep' was going to have a screw propeller, driven by a power take-off, operating in a dedicated tunnel faired into the rear end bodywork, as well as a proper rudder.

[2] The Marmon-Herrington prototype's hull formed an integral unibody structure, created by cutting shapes out of steel sheet and welding those together.

As a consequence, a low freeboard in the water meant that the GPA could not handle more than a light chop or carry much cargo.

The GPA's intended use of ferrying troops and cargo from ships off-shore, over a beach and continuing inland, was therefore very limited.

The most famous one was during the 1950s when Australian Ben Carlin (1912–1981) sailed and drove a modified Seep, that he called "Half-Safe"[10][11] on a journey around the world.

[12] A young American couple, Helen and Frank Schreider, converted one which they called "La Tortuga" and traveled from Los Angeles to the Southern tip of South America (1954–1956).

[13] World War II British paratrooper veteran Lionel Force purchased a GPA from Levy's Surplus in Toronto, Ontario, Canada, and called it "The Amphib".