Actually the nickname came from a cartoon character in Popeye the sailor man.
His pet was a "Jeep"(cheetah looking thing) that could disappear from one spot and re appear in another...anytime..any place.. Hence the Jeep being the mostreliable vehicle in WWII that could "go anywhere,at any time" they nicknamed it "Jeep"
G.P. refers to any vehicle used for running men,supplies,etc. trucks included.

Other useless Jeep facts-
Jeep is the ONLY vehicle that was made by all the major american auto manufacturers:
Willeyz-overland,Ford,GM & Dodge.

All of the modern day SUVs were "inspired" by the Willeyz 4wd powerwagon,the first vehicle with reliable 4wd.The willyz "stationcar" was the first SUV style vehicle.

WWII Jeeps could float,and was so light that 6 men could pick it up and move it if it ever got stuck.

The distrubuter points on the Willeyz Jeep were the same on the Harley motorcycle of the day.