The 100% completion checklist is viewable inside the game's pause screen, under Progress although some will be blocked out until discovery. In order to attain 100%, you will need to complete the following:
Missions / Events:
- Complete all 107 Main Story Missions
- Complete 10 Stranger Mission Strands
- Complete five Bounty Hunter Missions
- Have 25 Chance Encounters
- Survive a Gang Ambush
Collectibles:
- Find a point of interest
- Find the 9 Graves
- Collect a Complete Card Set
- Complete "A Test of Faith" (30 Dinosaur Bones)
- Complete "A Fisher of Fish" (13 Legendary Fish)
- Complete "Duchesses and Other Animals"
- Complete "Geology for Beginners" (10 Rock Carvings)
- Complete "A Better World, A New Friend" (5 Hunting Requests)
- Complete a treasure hunt chain
- Find 20 Dreamcatchers
Compendium:
- Discover 50 Animals (Study)
- Discover 10 Equipment Items
- Discover 10 Fish Species
- Discover all Gangs
- Discover 10 Horse Breeds
- Discover 20 Plant Species
- Discover 48 Weapons
Player:
_ Reach max Health, Stamina and Dead Eye
- Achieve a Level 4 Bond with your Horse
- Complete all 9 Challenges
Miscellaneous:
- Discover 5 Shacks
- Kill 5 Legendary Animals
- Play all 4 Table Game Once
- Interact with 5 Special Characters
- Take a bath (in a hotel)
- See a show
- See a live Theater Show (Rulear in Saint Denis)
- Craft 1 Cooking Recipe, 1 Tonic, 1 Type of Ammunition, 1 Hunting Item, 1 Horse Care Item, and 1 Weapon
- Complete a Home Robbery, a Shop Robbery, a Coach Robbery, and a Train Robbery