For me, after trying all other recommendations, the one that worked was the Mercedes-AMG GT3 ‘20. On the first attempt, after surprisingly not making any major mistakes, finished 2nd place. Tried a couple more times but the GTR GT-500 ‘99 in front was impossible to catch. So I exited the race completely, rejoined and got a different grid of cars.
What I found is that any mistakes on lap 1 before the straight after coming down the mountain, and the race was over. So I restarted the race a few times until I nailed that first lap segment.
What worked for me was, RH tires, FM1 on lap 1. FM2 laps 2-5. Pit at the end of lap 5, just for fuel. FM1 for the rest of the race. I finished the race with 0.3 laps left of fuel and just over 1.5 seconds ahead of Ford GT LM Spec II Test Car in 2nd place.
The AI are very aggressive. Keep your car right in the middle of the road in the straights and avoid any contact near the back sides of your car or else you’ll go spinning and game over. Break on time and avoid hitting the barriers in the mountain.
Overall, this race was amazing. I started hating it, but it was really a great, difficult challenge.