Manufacturers in GT1: Toyota, Nissan, Honda, Subaru, Mitsubishi, Mazda, Aston Martin, TVR, Chevrolet, Dodge. Needless to say, there were a lot less Kei cars (those producng about 63hp in stock trim) and less sedans. A lot of the JGTC and pure racing cars weren't in GT1.
Some cars have insane amounts of power in GT1 (Megaspeed Cup anyone?), and not in GT2. There's about 180 or so cars in GT1, and about 600 in GT2...do the math and making a list right here is a ridiculous waste of time.
I think the focus of GT1 was to buy a car, soup it up, modify it, and race it from bone-stock Sunday Racer to an eventual racing's-my-only-profession machine. The flow of the game is more along those lines than GT2 is.
GT2 put more of a focus on going into higher echelons of racing, in my opinion, and GT3 really ran with that concept to pure racing game. It appears that GT3 is half racing cars, from the little I've seen.
I think GT was really a novel concept, and I myself wasn't impressed with the game the first time i played it. I was more interested in racing real racing cars on real courses, not the fantasy that it was. But after getting hooked on GT2 like a crack fiend, I see that the Gran Turismo series is really 3 games in one, a racing game, a fantasy game, and a true first-person experience.
I think that GT's popularity really came out of left field, and surprised Sony quite a bit. Two friends of mine that never gave driving games a second look were both GT1 (and eventually 2 and 3) gearheads before long, asking me about things like
"What's a racing line?" "How come Skylines aren't sold in the U.S.?" and "NOW I see why you're hooked on racing games..."
So to answer a question, here's some GT1-only cars:
Nissan Silvia LM
Toyota Chaser LM
There's also some cars with tremendous amounts of horsepower. The Supra RZ, GT-R, Cosmo, Soarer, 3000GT/GTO, etc.