R32 GTR came out a few months after the R32 GTS coupes and sedans. All skylines, regardless of generation are based on a passenger car platform. The first skyline ever made was made as a 4 door passenger car. It just *happened* to win a couple of races so they decided to make a coupe in the next model that was developed.
To classify as a supercar it has to be a bespoke platform... ie designed from the ground up with only one purpose... to be a supercar, it can't have family car variants etc, which is why an evo, skyline etc aren't supercars. This is why the Z platform is considered the "true" sports car in the Nissan family, whereas the skylines are considered sports sedans... When a true supercar is made, you get to customise everything from the trim of your choice and colour (not just pick one from a bunch that the factory offers) to number of gears and gearing, seat shape etc. Basically handmade for custom one off order.
The closest things to japanese supercars are NSX's, GTO's and 300ZX's (all 3 of which were coincidentally sold worldwide, including australia and america).