First, remember that civilian auto production STOPPED for the duration of the war to enable the production of war-material...
Regarding the GP-1 (G" for a "Government" type contract and "P" commonly used by Ford to designate any passenger car with a wheelbase of 80 inches.): it was CERTAINLY the most produced... over 600,000 were build during the war.
Compare that with the Volkswagen Kübelwagen (German Jeep equivalent) of which only 50,000 were built. The Krupp Protze (German light-truck) numbered only 7,000, and the Opel Blitz was produced in roughly equal numbers.
The only US cars really produced were the Staff-Cars:
http://news.webshots.com/photo/1519988276056627778ZgIfRf
http://www.robertsarmory.com/staff.htm
http://www.robertsarmory.com/staffnvy.htm
EDIT: ONLY 700 VW Kafer (Beetle) were produced between 1936 and 1945.