You don't need any AKC registration to do the CGC test, any dog can take it as long as they have the vaccinations required.
However if you want the CGC *title* (as in, have it listed with AKC after your dog's name officially) I think you do need to have a PAL, AKC registration, or Canine Partners... You also pay extra for the title. However I think if you pass the CGC before you sign up for PAL you can still get the CGC title later.
If your dog is not registered, you still get a certificate that your dog passed.
Bianca HIC CGC TT (GSD) 4/24/04-10/23/12