I'm still confused when reading GnuTLS/Nettle implementation about Camellia-GCM.
Does Camellia-GCM use Camellia-CTR to create GCM Hash?
GCM is a mode of operation for a 128-bit blocksize block cipher that turns it into an authenticated encryption scheme. It doesn't care about whether you're using AES, Camellia, Twofish, Serpent or MARS. It will work the same way for all of these block ciphers. So yes, Camellia-GCM uses Camellia-CTR internally in exactly the same way AES-GCM uses AES-CTR internally. with Camellia getting the exact same inputs as AES.