Using an emulator you can see into the memory values of the PSX to see eventual differences in the 2 cars in the game.
Comparing the data of the 2 cars I see:
- the code for the suspension of both is different
- tyres are different too but I can't check dimensions as code has nothing in common with their actual specs
- the ride height code (and I'm sure of that because I used it in tha past) says the exact same value, 8C-8C, which means both cars are 140mm from the ground: probably the code behind both is the same, the only thing that changes is the name and the price.
P.S.: The ride-height code is the same even for the ZR-1, which I don't know if it was lower or higher than the original. However, pick the Coupè, it costs 4k less and is free from the terrible rims and from the sort-of-racecar paint scheme of the Grand Sport.
Edit: I checked Carfolio which usually comes with good data. The 1996 Corvette was 107mm high from the ground, and the GS appears to have the same datas. Also I didn't find differences between the wheel dimension. The big difference which the games ignores is that the GS has 333HP LT-4 engine, which is quite different from the stock LT1, which delivers 300HP. The Coupè had the LT1, instead in the game it's got exactly the same amount of power as the GS and even the same weight. Have we discovered a sort of mistake noone had noted?