The current PS4 trophy percentage for the platinum is 0.1%. This is largely meaningless as it takes in every PSN account that has the game added to their profile. This will include people with multiple accounts, people who played the game once because it came bundled with their PS4 and never touched it again, and people who took one look at the game and immediately started shrieking MY SINGLE PLAYER MODE. Even with that in mine, 0.1% is the lowest the percentage can go, so it's nowhere near precise enough for a game with millions of players the way GT Sport does.
Trophy tracking sites will always have higher completion percentages for games and trophies. This is because you have to put your PSN ID in there manually, and such sites are predominately used by people who make a point of earning trophies.
Currently, PSN Profiles has it at 0.72% of 141,500 players:
https://psnprofiles.com/trophies/6806-gran-turismo-sport
Truetrophies as listed is 1.2% of 34,000 players:
https://www.truetrophies.com/t190763/gran-turismo-platinum-trophy-trophy
PSN Trophy Leaders has 1.4% of 59,800 players:
https://psntrophyleaders.com/game/view/gran-turismo-sport-ps4
For reference, the Gran Turismo 5 platinum on PSNP is 0.62% from 670k players:
https://psnprofiles.com/trophies/460-gran-turismo-5