Information regarding this model is a bit scarce and confusing due to some models using the same designation.
There’s either one or two of these GTO prototype models in existence, the Silver one is definitely confirmed and the yellow one may be another GTO or it may just be a coupé race car.
The Coupé race models of the Caterham 21 are sometimes referred to as the 21 GTO but they’re not the same as the Prototype GTO model aside from both having a hardtop.
There were open-top race 21s as well and sometimes the GTO race models were called 21c’s to differentiate them.
The authentic GTO prototype had a 1.8L Inline-4 producing around 230hp while the 21c’s had a much wider range of engines available with one even being modified with a 2.4L Caterham V8 engine.
Unfortunately the current whereabouts of the Silver GTO are unknown but there are groups dedicated to Caterham models and specifically the 21 in the UK who would likely be interested in seeing it found again.
Also on a sidenote this feature is so underutilized unfortunately.
It’s mainly cars from Forza Motorsport 7 and before that have the bodykit components of other models in the range, although normally we get the top trim with the bodykit for a lower trim.
Like the '03 Celica SS-I having the Celica GTS bodykit and some of the Audi RS models with the S/A bodykits.
Not sure if it’s just a facet of licensing though and essentially trying to combo two different cars for the price of one from a manufacturer which makes it more difficult/expensive.