Are we talking about the GTWC in B-Spec? If so, I've done it with, IIRC, the Black GT-One. It wasn't terrily easy, judging by several pages of notes, but not terribly difficult either. I think it came down to tyre choice and pit strategy....
No, The GTWC is in the Professional Hall. This is about the FGT World Championship.
You know (or may not), the championship with 15 races, that will be
60 laps - Tokyo Route 246
127 laps - Twin Ring Motegi (Super Speedway)
74 laps - New York
77 laps - High-Speed Ring
62 laps - Grand Valley Speedway
23 laps - Circuit de la Sarthe I
78 laps - Cote d' Azur
113 laps - Seoul Central
76 laps - Infineon Raceway (Sports Car Course)
85 laps - Mazda Raceway Laguna Seca
64 laps - Twin Ring Motegi (Road Course)
15 laps - Nurburgring Nordschleife
64 laps - El Capitan
67 laps - Fuji Speedway 2005
53 laps - Suzuka Circuit
The only AI competition you will have is 5 other Formula Gran Turismo '01 (the car that looks like formula one cars).
And on the Pal version you win 1st: $ 100,000 2nd: $ 25,000 3rd: $ 10,000 4th: $ 5,000 5th: $ 2,000 6th: $ 1,000
Championship Winning Prize: $ 3,000,000 and a 1989 Mercedes-Benz Sauber C9 Race Car
However, to B-spec this (FGT-WC) you can (reasonably) easy do this with a B-Spec driver with a skill level of 6500 or more.
Ideally you would want more skill than that, but this could do it with some management.
If and When I B-spec I always have the overtake ON. Other wise I might forget to put it back on and end up last.