Adaptive vsync is just that, adaptive. It will allow slight variation as long as the frames are synced. When you just use standard vsync with a 60Hz panel, if you can maintain 60FPS+ then it will limit the framerate to 60FPS, if you drop below 60FPS it will run at whatever framerate the computer is capable of.
So, say for instance, the game is running at 60FPS+ most of the time, but drops down to 52FPS. With normal vsync, the times when the framerate dropped to 52FPS vsync would cause the framerate to go down to 30FPS. However, with adaptive vsync, the times when it drops down to 52FPS it would continue to run at 52FPS. When you have adaptive vsync set to half refresh mode that limits things to 30FPS, but again because it is adaptive it allows a little variation as long as things are still pretty much in sync with the monitor.
But like I said, if you definitely want 30FPS no matter what, create a custom resolution with a 30Hz refresh rate and select that in GTA(or any game) and use traditional vsync.