No, don't use GCFscape that's only really needed if you need to extract files from the GCF to make changes to them (having a file in a GCF is ignored if it's extracted and in the same path in windows explorer). All you need to do is download the GCF and put in the steamapps folder. Patch steam with undead and run it using a blank account. You should see the game there and will be able to launch it. If it's a game like counter strike source you'll need the most updated client.gcf as this is a gcf that cannot be download with cracked steam (correct me if I'm wrong).
Once you have put the GCF in the steamapps folder just restart steam and everything should be good.