True. A full-time Forza player who transitions from Forza to GT could struggle for a while due to the difference in physics and controller settings, though the same could also apply with the GT to Forza Transition too.
Take me for an example, not so competitive but very much a veteran GT player (played since the original GT on the PS1). I decided to buy a Xbox 360 so I may experience Forza, and I was not that good when I first started.
However, after a month of racing AI (with and without driving aids), I understood the difference in physics and became good enough at it to compete with other players. I usually finished between 4th and 7th against Forza players, yet my best finish was being the race winner in a 10-lap C-Class vans-only race with all Driving Aids except ABS disabled at Suzuka Circuit in a tuned AWD Ford Transit with a personally-made "Spiral Knights" livery (it was done a week ago, on the last day of my 12-month Xbox Live Gold membership).
Maybe Slap will eventually get better at Gran Turismo, we'll just have to wait and see.