GOLD (abbreviated in-game as G) is the main currency used in the Underground.

The protagonist gains GOLD primarily through winning encounters, whether that be through killing the monsters or sparing them. Fleeing a fight is not considered winning and, thus, does not award any money, although money can still be won if another monster was spared or killed in the same encounter. Killing monsters usually awards more GOLD than sparing them (with some very few exceptions). Depending on the monsters fought and the actions performed in an encounter, winning an encounter can award anything from nothing to more than a hundred GOLD.

Sources

Other than through participation in an encounter, GOLD may be accrued through the following means:

Transactions

GOLD may be exchanged for these various goods, services, and effects:

Items sold by vendors (funding the Temmie Shopkeeper's college education is listed as an item).

Thundersnail games (10G per game).

A stay at Snowed Inn (80G per overnight stay - this is refunded as the protagonist does not stay overnight).

A stay at the MTT Resort (200G per stay - no room key provided).

Muffet (spending increasing amounts of GOLD is an ACT option during the battle with her that lowers her attack for the turn).

A gift for Gyftrot.

Trivia