Gold pieces are taken to be the universal currency of role-playing games. Note that you can't MAKE MONEY FAST by just cutting them into smaller "pieces"; as with the U.S. nickel, GP are of a fixed size and value (thanks neil), but why does the value of the GP vary so wildly from one game to the next, even within a series? (Yes, I know, Canadian dollars.)
Plot holes like these are why many RPGs have gone to other currencies like nuyen (Shadowrun), gil (Final Fantasy series), what appear to be pesos (Pokemon), etc.