But the chances are that the best GT driver in the world is far more talented than Michael Schumacher....
If by GT racing you mean sports car racing--i.e., the racing of the likes of the Minolta, R8, Bentley, etc.--I beg to differ. In the many years I have been a fan of auto racing, F1 drivers, even some of the also rans, let alone the likes of Schumacher, consistently proved themselves to be excellent GT drivers also. In fact, many world champions have gone on to distinguish themselves in GT racing after retiring from F1. One example is German driver Hans-Jochaim Stuck. He never really distinguished himself in F1, but is among the most accomplished GT drivers. I was at Sebring in the 80s when he arrived from Germany, got in a Porsche 935, and set a course record to put the car on the pole, and the last time he had driven Sebring was something like 10 years before. Amazing!