If yo have solid grey interior you either
A: Don't have a GN but have the rarer WE4
B: Some one has swapped the 2 tone interior out.
Are all the correct emblems on the car?
The ONLY marking on the car that can denote a true GN is the WE2 code on the sticker on the trunk lid. Even then, someone could have swapped trunk lids.
Also, the dash plate (above the glove box) will look like brushed dark grey metal, a non GN had a Grey framed black plastic piece there...these could have been swapped as well.
Look for the code on the trunk lid WE2...
In the end though, a turbo Regal is a Turbo Regal