According to GTA Forums, and reported by VGA247, a group of modders have brought the entire map of Red Dead Redemption to Grand Theft Auto V on PC.
The mod is called Red Dead Redemption V, and the modders are aiming to make it "as close to the original as possible." The team are using the Xbox 360 version, and are remaking large portions of the map, with plans to also add "Parallax Occlusion Mapping and Tessaslation where applicable."
The team makes it clear that they do not condone its use in GTA Online, although they plan to add multiplayer support for third-party clients such as FiveM\FiveReborn. A beta test is planned for summer, but if you can't wait until then, you can check out the screenshots below to get the excitement going.
Is Red Dead Redemption's map one of Rockstar's best?