The Jack Hall Gang Treasure Hunt is one of the first that players can attempt to complete, as it is available right after starting Chapter 2, the very first story chapter where you are able to travel wherever you want on the map. This hunt won't bring you far from the Valentine area, so you don't have to worry about having to make a long trip to continue the story if you're tackling this treasure hunt early in the story.
In case you already completed this treasure hunt, you may want to check out the Poisonous Trail Treasure Hunt or the High Stakes Treasure Hunt. Both will take you a bit farther from Valentine than the Jack Hall Gang hunt, but they are just as rewarding, since obtaining and selling Gold Bars will net you enough money to buy a good horse and weapons early on.
The first map of the Jack Hall Gang treasure hunt can be obtained as soon as Chapter 2 begins. The map is sold by a treasure hunter called Maximo, who can be found on the ridge near the river to the northwest of Flatneck Station, north of Bard's Crossing.
Interact with the treasure hunter to have the option of purchasing the map. In case you are trying to be the baddest outlaw in New Hanover, you can just refuse to purchase the map and kill the man to get it for free. Doing so will cause a drop in your current Honor rating. If you, for any reason, don't get the map from the dead man, or if you simply choose to ignore him, you will be able to purchase it from any fence later on.
The first map will lead to Caliban's Seat, which is located to the south of Valentine and to the north of the "N" in New Hanover on the map. Reach the top of the rock formation by heading to its eastern side, and follow the narrow path on the rock to reach a hole, where you will find the second map.
The second map will lead you to the north, as you will have to head into the Ambarino region. The area depicted on the map is Cotorra Springs, which can be found to the south of the gap between the "A" and "R" in Ambarino on the map. The location is easily found thanks to the in-game notifications, as you will be notified that you have entered the territory of a Legendary Animal, the Legendary Wolf.
The beast, however, will not come out if you're not following the usual procedure for hunting Legendary Animals, so you can continue hunting for treasure without having to worry about anything.
Once in the area, you will find a circle of rocks that is hard to miss. Head over to the one in the center and check it out to find the third map.
The Jack Hall Gang treasure hunt third map is also the final, so get ready to get your hands on the treasure the gang managed to keep hidden for so long. The map will lead you to a lake in the Ambarino region, O' Creagh Run. The lake is found right below the "O" in Ambarino on the map, so it's extremely difficult to miss. If you're far into the story, a cabin on the lake is also the location of one of the stranger missions.
To get the treasure, you need to reach a small rocky island in the middle of the lake. Check under the rock to get two Gold Bars, which you can sell to any fence for a tidy profit.