Unless I'm missing something, the book doesn't ever mention how to get loot (only how to use it), nor does it mention what are the benefits, or purpose of Stygian Locks, that PCs are apparently presumed interested in acquiring.
The loot is the guns you collect after quests, when rolling up new weapons you can "burn" any guns you have to add to those rolls (1 gun per +1).
And the locks, at least from my perspective, appear to be similar to checkpoints. You need how ever many locks the gm decides in order to get the lost treasure and thus beat the game. Any benefit they may give would be narrative and up to gm discretion.