The first real racing game ever made.
In the simulation mode you can have your own collection of cars and tune them and race in different events. When you start you have enough credits to buy a used car. There are mostly Japanese cars in this game and only a few American and European. After you buy a car you can start racing in beginner events and then working your way up. Some races have regulations like front wheel drive only or only a certain type of car is allowed etc. When you get to higher level events you will need a license to enter. License tests teach you driving skills like accelerating and braking, cornering, etc. You will win credits from races and you can buy new cars and upgrade your cars with your credits. You can upgrade things like tires, brakes, transmission, turbo, and you can even put a racing paint job on your car. There is even a car wash which will increase the value of your car if you wash it.
In the arcade mode you can choose any car or track and just race for fun. The tracks are pretty cool, I don't think there are any real tracks in this game. The High Speed Ring and Trial Mountain are the best tracks. You can save replays of your races for both modes.
The graphics are pretty good for an old game. The sound effects are good, most of the cars sound different and the sound will change if you put a new exhaust on. Good amount of detail for a PS1 game. The soundtrack is pretty decent. I think the only two licensed songs are "Lose Control" by Ash and "As Heaven is Wide" by Garbage. Every manufacturer has a different tune, which is cool.
Gran Turismo is a fun, fast racing game for the PS1, and it features plenty of licensed upgradeable cars. It's probably the first racing game to have these features and that's why I think it's the first real racing game ever made.