A group of modders has built a tool called .White that is letting them port the map from Red Dead Redemption into Grand Theft Auto 5 and populate it with new gear.
Not only will you have the whole world of Red Dead Redemption to explore, the team plan to make it available for use with multiplayer, too.
While a trailer is on the way, the team shared some screenshots of the mode:
The team is currently looking to expand, bringing on programmers who can help reverse engineers the different file formats used in the game.
As for how far along the mod is, apparently it's been in development for two years already and the team hopes to have the first beta build out this summer.
You can read more about it over on the mod's page.