It's not a GT3 car though; the GT3 car was the GT3 Cup (then the GT3 R) which retained the rear-engine layout while the RSR was famously mid-engined.
Ultimately the "gaps" and missing brands are all down to licensing. The Corvette, as noted earlier, hasn't ever been in the game as a car newer than the C5.R (which ran from GT3 in 2001 all the way through to GT6). It requires a separate licence, from Corvette Racing - Pratt & Miller - and isn't covered by any agreement with Chevrolet; for some reason Pratt & Miller and PD can't come to an agreement (the C5.R somewhat proves that each brand knows the other exists...).
Other brands may have exclusive agreements with other titles. It's likely that Kunos sweeps up a lot of GT3 cars under ACC - the official game of the GT World Challenge - and that could be why we have basically three "new" GT3 cars (and a handful of older cars) in GT. I suspect they've specifically licensed GT3 cars outside the scope of the SRO, which is why the real ones are N24 cars (and winners) and GTE from IMSA/ACO. There's still "gaps" there if that's the reason, but just because the 488 GTE is licensable it doesn't mean Ferrari (or AF Corse) are willing to do so at the right price.
No matter what we say, there'll be cars PD just can't get hold of - such as one American brand which values itself highly despite not having any particular laurels on which to base that, and which has spoken openly about not being in GT because the price isn't right for them.