Rockstar, the creators of GTA V, have built their own proprietary game engine, known as, Rockstar Advanced Game Engine (RAGE).

I can assume they built the game engine off a mix of C++ and C, and then continued to use C++ (maybe JavaScript with V8, but not likely) on top of their game engine to build features specific to each individual game.

After doing a fair amount of research on the internet, I can conclude that my initial thesis was right, but that they probably also use C# in their multiplayer servers. In what language was GTA IV programmed in? - Help & Support

For more information on how they modeled the city check out Manish Sharma's answer.

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