I've seen this discussion before. Although G.P. as 'general purpose' is a nice idea, it seems that it is a mis-conception.
"'G' was used by Ford to differentiate between vehicles produced for the Government and for civilian use. (A= Passenger car, B= Bus, C= Commercial vehicle, etc.).
Letters were used at Ford to differentiate between different models with different horsepower ratings, wheelbase measurement, etc. 'P' just happened to land at the "80 inch" spot, which is the wheelbase of a jeep."
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Someone also suggested "Just Enough Exchangeable Parts"

CHEERS

Nigel