Im not claiming to know, just passing on information I have read elsewhere.
"One common misconception is that every Impreza between 92-01 were "GC8's". The code breakdown is as follows; G - stands for Impreza, C - sedan, M - coupe, F - station wagon, The 8 stands for the 2.0 L turbo motor. So really, there are many combinations. A coupe STi would be GM8, sedan GC8, wagon GF8"
Its from this site:
http://www.subarujdm.com/jdm_impreza.htm