Actually, I believe the "GCN" is in reference to the model code of the product, much like the Game Boy Advance is referred to "AGB" (obvious), the Game Boy Color is referred to as "CGB" (also obvious), and the original Game Boy is referred to as DMG (Dot Matrix Game). These are the codes used when a model type is reference in a product code, so typically that is why they would expand to other forums.
I am not sure if this is 100% correct or not, but I believe that it is.