Grand Theft Auto V (GTA V), developed by Rockstar North and published by Rockstar Games, uses the proprietary game engine known as RAGE (Rockstar Advanced Game Engine).
RAGE has been used in many of Rockstar's titles since its inception, including games from the GTA series, Red Dead Redemption series, and Max Payne 3. Its capability to handle large and immersive open-world environments with high detail levels makes it a suitable choice for games like GTA V.
However, the use of RAGE alone doesn't account for all of GTA V's technical achievements; the game also integrates components such as Euphoria (a dynamic motion synthesis system developed by NaturalMotion) and Bullet Physics Library to handle realistic character movement and physics simulations, respectively.
It's worth noting that despite being powerful, RAGE is not available for public use or licensing, which means it cannot be used by other developers outside Rockstar Games. This contrasts with engines like Unreal Engine or Unity, which any developer can use to create games.