Most if not all the shootouts in the game didn't happen, Silas Greaves didn't exist and we have exactly zero proof that there ever was one single duel in the Old West as we see them in the movies. Basically, except for the names the characters are based on, everything is made up.
As with both the original Call of Juarez and Bound in Blood, the team did a lot of historical research during preproduction. However, as with both of those games, they didn't allow historical actuality to slavishly dictate the entire content of the game.