The 1998 car posted slower laptimes but I can't find anything different as far as actual specs so i'm not sure if it was just tweaked abit from the 98 car as the aero is very slightly different and maybe gear ratios. Assuming they add the 1999 car instead it would be pretty easy to BOP it into Gr.2 with added weight and lower downforce levels.
It's really the most hard to predict car when it comes to categories if it comes to GT7.
The 1998 version qualified 2nd behind the Mercedes CLK-LM, so that version would surely have to be in Gr.2 since the CLK-LM in the game is a Gr.2 car.
However, the 1999 version was tuned to the new LMGTP regulations, and while I can't exactly find the specs about it, the lap times don't lie.
The 1999 GT-One was over 6 seconds faster than 1998 one around Le Mans, same layout and ambient conditions not too far apart. And it was competing against the new LMP1 cars like the BMW V12 LMR.
The 1999 version is faster than any Group C car in the game. In fact, in real life, the only Group C cars faster than the 1999 GT-One are the 3.5l V10 Group Cs from 1991-1993 like the Jaguar XJR-14, Peugeot 905 and the Toyota TS010.
And also, it's more than likely that if this car ever comes to the game, it's the #3 Esso Ultron sponsored 1999 car that will join the game considering that's the car that's been in the game from GT3 to GT6.
So basically:
1998 GT One = Gr.2
1999 GT One = Gr.1
Any LMP top class car from 2000 and above is pretty much guaranteed to be a Gr.1 car if it joins the game (Audi R8, Bentley Speed 8, Audi R10 Tdi). The 3.5l V10 Group C cars as well since they are an order of magnitude faster than the "classic" Group C cars and those in GT7 are Gr.1.
Gr.2 is reserved for "current" gen DTM, post 2008 GT500 cars and 1997-1998 FIA GT1 cars (Nissan R390 GT1 would also be a Gr.2 car). And I don't see any other class really fitting in it.
Or rather, mixing GT500 cars with late 90s GT1 is already a recipe for disaster because you really can't make a BOP balanced with these two cars. GT500 and DTM is fine, they are pretty much similar, much like how GT3 is to GTE.
Gr.1 is even worse though. 80s Group C + mid 2010s LMP1-H + early 2010s LMP1 + Withcrafted VGTs ...
I know PD doesn't make race events with the cars actual categories (Gr. C, LMP1, etc) because of licensing issues but I believe there could've been a better way to categorize these cars.
Damn, all this talk about all these amazing prototypes... I know GT7 isn't really lacking on them right now, in fact, it has quite a nice list compared to most games, but there's so much more to be added from both the past and present.