I go for gold in the arguably hardest challenge in GT Sport.
I did this live, so I paused a couple of times to answer a few comments.
You want to start off with Race Soft tyres and put your fuel mapping up to 6 so you only have to pit once. When pitting right after you pass the 15 min mark, change tyres (Race Softs) and refuel for at least 10 laps, but at the most 11. You should try and get a 1.24 lap or faster every lap to be in pace to win, with almost no room for mistakes whatsoever.