GP is short for gold piece typically, so it's a safe bet that's what they meant. However, it's supposed to be gil, GP is simply from translation. Most re-translated earlier games have switched back to gil.

One thing I would like to add though, that occured to me about the space issue of mass amounts of coinage. That does make sense, but most FF games allow the characters to carry 99 of every kind of the 255 items. 99 of one kind of sword is bad enough, but 99 of every different kind of sword would be impossible to carry around, so gil is the least of the 'impossible to haul mass amounts of' worries.