Hey, thanks for asking!
With multiple players, everyone who defeated the map's boss compares their gold at the end of the game. Whoever has the most gold wins.
When playing solo, simply try to collect as much gold as you can and defeat the boss in the allotted number of rounds. If you like, you can record your result and try to beat your previous best on each subsequent playthrough.
You can find these rules on page 3 of the rulebook under "Game Modes".