Well I think that, to make one's self-opinion on realism, several parameters should be studied before starting comparison. I'll just give my own opinion here, and of course I don't pretend it is the right truth. I'm gonna just talk from my own feelings and experience - after all, everyone is different so that's why there are so many opinions on this topic
Also, even if I've been seriously into car simulation for about a year (mainly offline, just joined NoGrip in December), I've been interested in simulation in general for many years now, particularly in flight simulation (Flight Simulator, IL2 Forgotten Battles, Pacific Fighters...).
First of all, GTL and GTR2 don't gather the same types of cars. The rides in GTL are old ones, many could be slightly closer to 'Mr Everyone's car', whereas in GTR2 you got hyper-sophisticated cars, runnings like rockets. So, about driving feeling, I guess it is tough to compare these 2 games. Even if, obviously, it can be just a matter of taste.
About GTR2/GTL and Gran Turismo, you should keep in mind that they work on different hardware : the first 2 ones on PC, the latter on PS2, which is a console. Here also, I think that first it would a matter of taste, of what you are looking for in a car game. Personnally, I find Gran Turismo series a bit too directed towards arcade-like driving. I haven't tried GT4 so I won't give any opinion about it. But I remember when GT1 had been released. And I remember the difference at that time : it really looked like a simulator compared to other games on PS1. But I remember that the gameplay, the feeling, etc was close to the arcade spirit in the long run. Why? Well I guess the answer is simple : people playing console games aren't the same than the ones playing on PC. They expect, in general way, different things. And marketing sharks have understood that of course!
Talking about graphics, difference between track versions : here again the hardware is different, and so graphics technologies are. This is due to electronics, but also about your own vision. Indeed, if you play a console game, you watch it on a TV screen, which got visual settings (resolution, colors, the way the picture is displayed etc) that are different from a PC screen. Also, on a console, you are sitting in your sofa, most of the time at a certain distance from the pictures displayed. On a PC, you are sitting at your desk, and your sight is closer to the monitor. That could explain different vision feelings.
I agree, in any case this will always remain simulation software, so you will always be as close to real as possible, but it can't be as good as real. If you want to know how it feels riding a Corvette, well go and drive one will give you the best. I guess the game hardware is important too in order to make one's opinion about the quality of a game. The more accurate a steering wheel is the closer you get to reality in a car simulation. I've personnally started with a rumble gamepad and recently got the Logitech Momo wheel and I can tell you that I saw the difference!
A good gear - here again it's matter of taste and money - can make our game more accurate. I give you a personnal example, nothing to do with cars, but with simulation though : I had played flight sims alot with a Microsoft Sidewinder joystick. It was very fine, I really enjoyed it this way. Last year I changed to another joystick, a HOTAS (Hands On Throttle And Stick) system, the Saiteck X52. I tell you what, it was like I was born again, lol! And I guess that it'll be the same feeling again if someday I can sit in a flight sim cockpit. I won't have to tweak and push buttons with mouse, or have a look to my flight instruments on my monitor, I 'll just watch the scenery on my screen, and operate in the cockpit like in real! Dream on, lol! I've seen such gear and this is bluffing! I've seen how it goes in several exhibitions (including car simulations too) and it's sure that the better your hardware is (and the richer you are in some cases!) the better you feel with your sim. And sure it will be the same feeling if smeday I change from Momo wheel to the G25.
Well I guess everyone has his own opinion about realism, and there are as many anwsers to this question as there are humans on Earth! What I can personnally say as a conclusion is that GTL and GTR2 are two fabulous sims, close together because of the way they are made, the gameplay etc, but also complementary (excuse my French, I don't if you say this word in English!) because they are in two different times in car racing history.
And whatever you think, don't forget the main thing : have fun and pleasure!!!