If I were to give a GT class designation, the NISMO would be GT2. Unfortunately, it would be on the slower side of GT2. You can't really compare this with a GT3 car, like the 911 GT3 or the Corvette Z06, since those had all the high-end natural grip and downforce. The GT-R NISMO, especially in markets outside of Japan where the N-Attack package rather wasn't available, and especially since the 7:08 lap time was done at the 'ring with track-only tires, was much more comfortable than any GT3 car.
Quite ironically, GT2 is more like GT4 than GT3. Likewise, the former GT1 is more like today's GT3. With Nissan, IT IS TRICKY. But you can never call this a GT3 car. It's a "soft" GT2 car like today's Maserati GT2 Stradale.
Its closest competitors in terms of driving dynamics would have to be the Ford GT and McLaren 675LT (or 765LT, depending on the model year) and Ferrari 488 Pista, but those are still way out of this car's league in straight-line performance.