A lot of people throw around GC8, BC/BF, GGA, GDB, etc, but often times people don't really know what they mean. These are chassis/engine combinations for various model codes of Subarus. Some of these terms are being used interchangably, when they must be uniquely identified.
When referring to a chassis alone, you use the first two characters of the model code.
1993-2001 Impreza Chassis:
2002 - 2007 Impreza Chassis:
2007 - 20013 Impreza Chassis:
Widebody hatch: GR
Widebody sedan: GV
1990-1994 Legacy Chassis:
(Yes, the C and F indicate the same chassis as Imprezas)
1995-1999 Legacy Chassis:
Outback Wagon: BG (Raised roof)
2000+ Legacy Chassis:
The third character refers to the engine that particular chassis was equipped with from the factory. The engine code itself implies engine size, configuration, and other things like radiator type and crossmember used.
Due to some confusion on the VIN versus the applied model code, the table has been revised to show Model Codes and VINs.
Applied Model Codes:
1.8L (NA-Impreza): 5 (ie: GC5 is a 1.8L Impreza Sedan)
2.0L (pre-MY01 WRX): 8 (ie: GC8 is a 2.0L Impreza WRX Sedan)
2.0L (MY01+ WRX): A (ie: GGA is a 2.0L Impreza WRX Wagon)
2.0L (MY02+ WRX STi): B (ie: GDB is a 2.0L Impreza WRX STi Sedan)
2.2L (NA): C (ie: GFC is a 2.2L Impreza Wagon)
2.5L (NA-Impreza): E (ie: GME is a 2.5L Impreza Coupe)
2.5L (NA-Legacy): 6 (ie: BE6 is a 2.5L Legacy Sedan)
2.5L (XT): 6 (ie: SG6 is a 2.5L Forester XT)
2.5L (STi): 6 (ie: GD6 is a 2.5L Impreza WRX STi Sedan)
3.0L (H6): 9 (ie: BE9 is a 3.0L Legacy Sedan)
VIN Chassis Codes:
1.8L: 2 (ie: GC2 is a 1.8L Impreza Sedan)
2.0L: 2 (ie: GD2 is a 2.0L Impreza WRX Sedan)
2.2L: 4 (ie: GF4 is a 2.2L Impreza Wagon)
2.5L (NA and XT): 6 (ie: GM6 is a 2.5L Impreza Coupe)
2.5L (STi): 7 (ie: GD7 is a 2.5L Impreza WRX STi Sedan)
3.0L (H6): 8 (ie: BH8 Is a 3.0L Legacy Wagon)
3.3L (H6): 3 (ie: CX3 is a 3.3L SVX)
Some of this information is trivial, but in some cases being specific will ensure the correct answer to your question.
Specific model code indication required:
If you are requesting fitment of a radiator into a GFA, there are two things to consider: GF indicates pre-MY02 Impreza chassis, while A indicates it is a 2.0L turbo engine. For a vendor to simply reply saying that, yes, they have a GC8 radiator ( when it is really a GC4/6 radiator) does NOT mean it will apply to your application. It does means it will fit the GC/GF chassis, however. Since you have a turbo engine (A), it means you probably do not want a radiator designed for a naturally aspirated engine (6).
Specific model code indication not required:
If you ask if a STi V6 drivetrain will fit into a GC8 chassis, then of course it will. That is the exact car the engine is in. However, if you are asking if it will fit into a GC/GF chassis (or maybe just GF4 specifically), then your question is less ambiguous as to what you are trying to do. However, it is obvious to see what is being asked.