It's from the VIN of the car, but not neciserilly on your VIN. I think that GC8 designation is the WRX sedan VIN description over in Japan for the WRX's that look like the 2.5RS. So people just call it a GC8, since it's easier than saying "the previous generation Impreza." Did that make any sense? Anyways, it's something like that.
The "GC8" is a model code from Subaru to distinguish the Impreza RS and Impreza WRX or just Impreza. In Japan, the WRX already existed with the 'prior model design' (GC8)...Unfortunately, it wasn't until the new age Impreza (WRX) came out here...but because in Japan, WRX always existed before it came to the US, the distinguish by codes between the two; thus GC8 - pre 02.