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.
I agree. Source code is not needed at all, especially for a game like GTA V with a very active modding community. People have added new textures, ray tracing and even made new games with new campaigns without any source code. Releasing the source even officially won't change or improve anything.
This makes it easier to intergrate with more extensive integration.
You have no idea how much easier it is to mod a game when you can look at the source code. GTA V has a thriving modding scene despite its source being unavailable for modders.