Yep, I agree with Wolf. GCCs are extremely needy little birds - I often compare them to cockatoos in that sense because, like them, they are not happy unless they can spend hours and hours and hours on their human. I have one now, a female named Codee and even though she has a boyfriend of her own (a severly handicapped male peachfront conure), she still spends two solid hours on me if not more. Lone birds need more than that to be happy. I had four, all of them rehomes and all given up because of their aggression and they all stopped biting once they got lots of one-on-one. They also need to fly (the only way they can get the 'ants out of their pants'

) and to eat a low protein diet (free-feeding protein food like pellets, seeds, nutriberries, avicakes, etc is a no-no with them) and this is not only because they are mainly fruit eaters in the wild but also because a high protein diet (especially if it's pellets made with soy) makes birds aggressive.
So my suggestion to you is to re-evaluate your husbandry (light schedule, one-on-one time, out-of-cage time, diet, flying time, etc) and see if the biting could not be caused by something that he/she needs but it's not getting.