"There are a few exceptions relating to participation in a CGC test. If the CGC test is given at an AKC show, the age requirements for the show apply to CGC also. Further, when the CGC test is administered at an AKC show, the test may be restricted to only dogs entered in the show or to purebred dogs."
Which can mean that it is sometimes difficult to find a CGC test with a mix. Depends on your area.