Of course, Rockstar uses its own internal technology, assets, development pipeline, and engine to make its Grand Theft Auto games, and the games are incredible for that- not using a generic engine, and instead using a development process that has been tailored around the requirements of the game, Grand Theft Auto games are always technically ambitious, expansive, and impressive, and visually stunning as well.
But, of course, there is the lingering question- what if it wasn’t like this? What if Rockstar did use Unreal Engine to make GTA? In case that is a question you have often pondered, here is your answer- a new open world project being made in Unreal Engine 4, called AQP City, which is based on GTA 5, got some new footage, which you can see above.
If you don’t think it looks particularly impressive, just remember that the game is in alpha, and being developed only by one person in his spare time. If it had had all the backing of Rockstar behind it, just think of how good it might have looked.