The full GTA V source code has been leaked The leak contains GTA V source code and stuff from Bully 2 and GTA VI Leaked in a discord server by a random British guy in the 360 modding community known to get sued by Rockstar multiple times
"Now i am expecting a open source version of gta to arrive soon on linux natively . Tired of playing supertuxcart."
Here is the source. Another one.
It's probably just the code without any assets (textures, 3d models, etc.). Code really doesn't take much space. You'd have a hard time actually building it to something playable as is, but this is still useful to modders.