To get 100% in the game, you will need:
- Beat the main story (obviously).
- Do almost all of the Stranger missions ("I know you" doesn't count).
- Purchase every rare weapon (LeMat revolver and Semi-auto shotgun from Escalera, Mauser pistol, Evans repeater and Carcano rifle from Blackwater).
- Get nine outfits. The Cowboy outfit, Duster coat, Walton's gang outfit, Bureau uniform, and Legend of the west arent required, neither all the DLC outfits.
- Do at least once the nighwatch job in MacFarlane's Ranch, Chuparosa and Blackwater.
- Do at least once the horsebreaking job in Ridgedwood Farm and Chuparosa.
- Obtain all safehouses. Most of them you need to purchase them, and others will be given to you as the story progresses. Pleasance house and the ones that you rent aren't necessary.
- Complete all gang hide-outs.
- Complete all challenges. DLC challenges won't count.
- Do all the bounty targets (20).
- Win all minigames. To get a win, you need to eliminate the other players from the table. For the blackjack, you need to leave the table with more chips than when you started.
- Discover all of the map locations in the game. You can either travel to them, or just buy a map in general stores.
For a more detailed info (and see the math behind all of this), go to
https://reddead.fandom.com/wiki/100%25_Completion_(RDR)