I guess you mixed bad track + bad car physics : not everybody is a physics expert and many mods (cars or tracks) are pure crap.
Moreover GTR2 is a GT game and not NASCAR where suspensions are not symmetrical so you must go in garage setup and untick box symmetric then spend lot of time to adjust separately left and right physics.
If cars fly then cgheight value is set too high in respective car HDC setup files so replace by
CGHeight=0.20
Do not exceed 0.30 or car will fly at each bump, it also depends on car speed, suspension and aero settings of course!
And increase rear wing aero downforces to limit car fly.
Depending on the track banking curve maybe car ride height is set too low so try to increase suspension height too.
GTR2 is a simulator so 50% of the work is to fine tune garage setup, this is clearly not the kind of game you can drive without spending hours in garage setup (unless you already know the car/track and how to set it up). If you prefer to select a car and make a race instantly without knocking your head on desk then you should consider to try EA NFS SHIFT 2 or any codemasters GRID which are excellent simcade with instant joy without taking care of garage setup.
GTR2 is actually rFactor 1.5 exactly between rF1 and rF2 physics, it was acclaimed as the best simulator in history back in 2007 with multiple awards winner. As any simulator, physics are considered as realistic as long as you keep values inside working window, so if car/track mod use bad range values then it becomes unrealistic.
Today you can attack GTR2 on graphics (old dx9) part but certainly not on its physics engine which is still better than many newer racing games/sims.