Official Rockstar storylines, characters, and other elements from the studio's 'established continuity' are off-limits.

Here’s a direct link: https://www.rockstargames.com/community-resources/mod-guidelines

The Game Developer story points out this will also apply to existing games, and includes directions to not mess with anything that affects continuity. It lays out the basics without getting into the weeds if you’re curious, that’s why I linked it primarily.