Gold or G is the standard currency in Chrono Trigger. In most RPGs, G (or similar abbreviation) stands for Gold. G is the accepted currency in most time periods, even in Prehistory and the Future, where it is called "shiny stone" or maybe questioned as money, respectively. The exception to this is in Prehistory where the Ioka Village Hut merchant accepts Petals, Fangs, Horns and Feathers in exchange for special weapons, armor, and headgear. Gold is accepted by most merchants in exchange for their goods. G is awarded by defeating certain enemies or found in chests.
Gold can be gotten in a variety of ways from defeating enemies, charming enemies items to equipping an accessory called Workman's Wallet to turn exp points into gold. Here's a list of places to farm money.