You can find treasure maps in various places. I came across my first one after I killed Flaco (sp) in one of the gunslinger missions. Each map leads to a location with a landmark then eventually the treasure. That particular hunt is called the Poison Trail Treasure hunt or something. At the end you go to a cave under a waterfall and find a bunch of treasure, including 3 gold bars:
https://www.usgamer.net/articles/22-...sure-locations