https://www.gamespot.com/articles/red-dead-redemption-2-tie-in-mission-discovered-in/1100-6455658/

Red Dead Redemption 2 is still a ways away--but Rockstar seems to be preparing for its release already. In fact, determined fans of Grand Theft Auto V have discovered references to secret missions in the latest update that connect to RDR 2. Some of them will even grant you in-game items to use in Red Dead. [Update: Rockstar has officially announced the crossover mission.]

The Guru Team, famous for solving the UFO mystery through mining game files, discovered mentions of RDR 2 deep in GTA V's Doomsday Heist update files. Specifically, they found descriptions of a Double-Action Revolver that'll be unlocked in Red Dead if you complete a corresponding mission in GTA.