There are loads of unused vehicles in the game. For example, the Prius R Mod and the VW Polo are both stored in the game's memory. At least a few dozen other cars are also playable using game mods.
Gran Turismo 2 was rushed to be released in time, resulting in a lot of cut content, like drag racing, and many unused cars.
One of the two discs that the game came with (the GT Mode disc) had a special "Scratch and Sniff" feature that, when rubbed, made it smell like a pit lane. This and FIFA 2001 were the only PlayStation games to include a "Scratch and Sniff" disc.