It wasn't too long ago that several community-made GTA Online "alternatives" were shut down by Rockstar.
Many players chose to enjoy GTA V as a multiplayer experience using special mods which allowed them to connect with friends on other servers. These allowed them to play in the single player world, or in special lobbies where certain other mods were allowed. Despite many of these alternatives being somewhat unstable, they were pretty popular in certain circles.

This new mod doesn't really fit the bill of "GTA Online Alternative", so is unlikely to be shut down any time soon.
Some of you young'uns may not remember, but there was a time when "multiplayer" in video games meant gathering at your friend's house, crowding the couch and straining your eyes at your corner of the screen.