Use the Gold Nugget to buy R.J. Earl’s Tonics, which is a transparency tonic that you will use to sneak int the hideout that belongs to the El Mustachio Grande’s Gang. This will lead to the final battle to capture the notorious bandit and outlaw.
How to get the Gold Nugget
Ride down to Rock Ridge and go speak with R.J. Earl. Open your backpack and use the gold nugget you found in the river. He’ll sell you all of his potions for the gold nugget. He doesn’t want you to sell anything at first, but when he saw that you have a nugget gold, he’ll give you his best tonic which is the R.J. Earl’s Tonics.