GFC or GCF?
GCFScape will allow you to extract the model files from the .GCFs, however you can't alter the actual .GCF.
You can use GCFScape to get the game's models from the GCF and alter them, then save them as a model with a different name (so the old and new ones don't have arguements), in case you don't want to make em from scratch
. After you have your new models, the process of getting them into the game is pretty much the same. Just pop em into the model directory, and you'll be all set. Of course, don't forget the textures.