Contrary to popular belief, the Buick GNX was never designed to be a "muscle car". It was relatively "cheap" (compared to other cars with similar performance), and it was different from most of the other US midsize coupes in that it was very fast with a very low-quality (even for its time) interior. It's still a personal luxury car, a middle-ground between the Mercury Cougar XR7/SC and Lincoln Mark VII LSC 5.0 HO.
Even with its very high "turbocharged" performance, the V6 engine is the personal luxury coupe's absolute biggest disqualifier from competing in the muscle category. To put up with forced induction from only 6 cylinders to compete with V8s is considered "cheating" in the Muscle Car World (MCW).
No matter the power or even the torque, a "muscle" car must have a V8, V10, or V12, and not even just that. The engine displacement must be greater than 5.5L if powered by an OHV, or 4.5L if powered by an OHC (using both of those numbers average 5.0L, which is typically the minimum for a muscle car to even compete).
A muscle car engine cannot displace "just" 4.0L (even if those four liters are packed with a V8) because that sort of displacement is synonymous with many 6-cylinder and some smaller European V8 engines, and a 4.0L V12 will either be weak in the torque curve or way too high-revving.