It's not perfect VR, as in this case the headset is simply recreating the mouse/thumbstick view from the regular controls (and not giving you full and proper VR support), but still. This motorbike ride looks like a lot of fun.
Falandorn, the video's uploader, says he managed it using vorpX (a program that converts existing games into VR games) and an Oculus Rift Dev Kit 2.
GTA V support was added as the latest update for vorpX; if you've got the game and an Oculus Rift, you can get the most recent version here.
Here's another video from ImmersiveGamer83, showing some pedestrian stuff. The VR starts at 7:42: