I must admit that when I first saw this in my book 25 years ago, I was flabbergasted. How could something that looked so much like my mom's Oldsmobile Cutlass Supreme be so quick?
Only 500 of these were built, and only in 1987. It was the last rear-wheel drive Regal ever built, so Buick wanted to go with a bang. Mind you, the original Grand National, which was what the GNX was based on, was already faster than the Testarossa and 288 GTO.
Here is how they compared:
GN: 0-60 in 4.9 secs, 1/4 mile in 15.7 secs at 97.3 mph
GNX: 0-60 in 3.9 secs, 1/4 mile in 13.4 secs at 105 mph
The book talks about how ASC and McLaren (of Michigan) were involved in the car's development, but it's not clear how they made the GNX more powerful than the Grand National.
Here is an old Car & Driver clip on the GNX: