As far as I know you don't lose any cars. They may have different names but I don't believe there are any missing in Geneva but I am not 100% sure.
Music and intro are normally different in the different versions. You can probably get the different intros on the net even on this site but music I am not so sure about.
There isn't any licensed music in GTC - it's all inhouse stuff.
GTC Geneva is actually badged GTC Tokyo - Geneva, so logically you'd think that it would be GTC Tokyo that's missing stuff, rather than GTC Tokyo - Geneva. It came out some time after GTC Tokyo.
I think that there are two Japanese versions of the game, cause on of them is GTC 2001, and I think the other one is 2002, someone correct me if I am wrong.
GTC 2001 Tokyo is missing all the European and American models. It only has Japanese cars in it. It was never released outside Japan (I have a dodgy GTC 2001, but my 2002 is Legit )
The only difference between GTC 2002 pal and Ntsc is that the PAL one is in English and the NTSC one is in Japanese.