Firstly I feel your pain. GCC can bite hard and it also hurts us emotionally!
I've gone through several periods of biting and even flying to me to bite me from my GCC. So I know you can work through this and get back to having your freindly bird again!
Something did happen to upset your bird, you aren't aware of it yet, but something happened from the birds point of view. ( Unless is only agressive and protective of the cage, that's normal and should be accepted that it their space) Also , work from the idea that the bites are your fault. It's how a lot of us look at it, and work from there. Parrots bite when we don't catch in to their other clues. Then we also do stuff that reinforces the biting, or show fear, uncertainty, anger, that makes it all worse.
It's extremely easy to accidentally make GCC afraid of hands. Chasing them to put them up, using your hand to shoo them away, accidentally brushing a link feather you name it.
Are you the only person in the house? Are there other pets? If you aren't the only person in the house, ask everyone else not to interact with the bird at all , while you are working to overcome this. Make sure no other person is going near the cage.
The good news is GCC seem to be easy to bribe! Especially fond of the safflower seeds!!! So start back at the beginning as though you just brought the bird home. Set up a treat only dish inside the cage, that you can put a treat in easily from out side the cage. Also set up a treat only dish on the top of the cage. Stand back from the cage and let the bird see you have a great treat, then say hello ( birds name) and walk up to the cage and put the treat in the dish , and then walk away. Do this many times during the day. Until the bird seems to be so happy to see you coming with the treat. Then do the same thing but give the treat by hand. If you have your bird out , do this with the treat dish on top of the cage. When all of that goes well. Do the step up get a treat and take a short distance away from cage to a stand or perch you have set up and put him there and give a treat. Let him.hang out there for a few minutes then go up say hello, give a treat, have step up and take back to the perch you have on top of the cage have him step off and get a treat again. You can repeat every few min for a short training period a couple of time a day. You are working on trust, and that you always mean good things.
If there are no other pets or small children in the house. Then when a bite happens you can say no and set them on the floor, wait a few seconds and pick them up and go on as though nothing has happened. Or if it hurt and you feel mad, give yourself a minute to calm down.. birds can read your energy, mood, and expression very very well!!!
This is a good link, I will also link my Ornithology thread, on page ten of my thread are several articles on behaviors.
https://petcentral.chewy.com/think-your-pet-bird-hates-you/
http://www.parrotforums.com/general...hare-discuss-scientific-articles-parrots.html