San Andreas Unity is an open source reimplementation of GTA San Andreas game engine in Unity.
This won’t be a complete reimplementation, but the focus is on gameplay features, mutliplayer, and creating a framework which will allow easy game extending and unlimited modding possibilities.
For more information about the project, read this wiki page.
VR Modification: When I discovered this project, I made a fork that would not only run on Android but also added some basic VR functionalities to it.
It kinda works. Place your GTA SA Files to /sdcard/gtasa on your quest and hit the Start in VR button and hope for the best.
Note that this is a very experimental and abandoned project. Do not expect something great here.