Neither Forza nor Gran Turismo even have realistic enough physics for someone to "know"
any track without having driven on it before. I wouldn't necessarily trust anyone claiming proficiency on a track based only on GT/Forza experience, even if they were a good driver in general. You can learn the sequence of corners but you can't learn much more than that.
As for the Nordschleife,
when I went in 2005 it seemed "larger" than in a videogame and at 18 years old it was pretty daunting for me, but the biggest difference was the obvious introduction of G-forces, particularly the incline at Fuchsröhre trying to pull your head into your chest.