Red Dead Redemption 2 is hitting PC on November 5th, and it will be available at Rockstar Games Launcher, Green Gaming, Epic Store, and Humble Store. It will also be a Stadia launch title, and it will reach Steam in December.
However, if you pre-order the game via Rockstar Games Launcher, you can now pre-load the game, becoming available to play right at launch. Another bonus of pre-ordering on Rockstar's platform is that you can also choose one free game from Bully (CannisCanemEdit), GTA III, GTA: Vice City, GTA: San Andreas, Max Payne 3, and L.A. Noire.
Before you pre-order, just make sure you check the requirements here.