I've never experienced life with a GCC so others may have more appropriate advice. Since I just posted this on a sun conure thread with similar issues, I will post my advice below (if this is puberty). I think it's universal:
1) Avoid situations that trigger aggression. It varies. You can train him out of these gradually - but later. During his first hormonal period, just back off.
2) He does something aggressive that you don't like, say to him calmly, but firmly, 'No.' Leave him somewhere safe (perch, cage, etc depending on your living situation), and calmly leave the room. Don't let him follow (I close the door behind me). Come back after 15 mins. This is the number one thing that worked for me (thank you, Kalidasa, forever). Just don't do it for extended periods of course
3) When you get bit, react the way birds do when other birds bite them (i.e., pained shriek, don't go over the top but do commit to it, no laughing or bad acting allowed haha). This will show your bird that he caused you pain and would help him understand that it's not a cool thing to do to his flockmate. I also would also point to the part he bit then gently touch his beak, saying 'No bite.'
4) Bite pressure train. Look up techniques on how to properly do this. It's very important because it allows your bird to express himself without resorting to a bite. Sometimes they just need to tell the human to move their grubby fingers out of the way.
5) Exercise. Flight training is great for this. Let's them burn all that energy and hormonal frenzy. Also builds bond.
What I do not suggest is to clip the bird's wings as a means to subdue him. This is a fast way to frustrate an already hormonally frustrated bird and lead to a host of other behavioral issues. I also do not recommend giving them Lupron (?) shots which is supposed to reduce hormones. That sounds like a good way to ruin the kidneys. All these were recommended to me by none other than a vet! Also do your best not to raise your voice or show agitation - it will always backfire. Not saying you do, but thought I'd mention it anyway.
Observe his health (appetite, weight, stool). If this change in behavior is not hormones, it could also be from something health related.
