It's probably because his surroundings are still so new. A lot of birds are so mild-mannered when they first arrive home because they're nervous and have no idea how to respond to all the new, unfamiliar stimuli, so they kind of just let everything happen and are very cooperative. After being in their new home for a few days, they start to lose their nervousness, but still don't really trust their surroundings or their new people.
I had pretty much the same problem when I brought my own GCC home earlier this summer. The very first day she was really quiet and cooperative, and on the second day I couldn't handle her at all because she'd bite hard enough to draw blood. The problem was that I was way overeager to be super-best-buddies-forever right away... which is scary and off-putting for other humans, let alone for a little bird.

Mostly it takes time. You really have to let the bird set the pace when it comes to bonding, while also setting reasonable boundaries for behaviour - things like preening and gentle mouthing are okay, but nipping and biting are not. With Akimi, if she preens too hard or nips I remind her to 'be gentle'. If she keeps doing it, or escalates to biting I put her down on her play gym or cage (but I don't necessarily shut her in). It's not a punishment, just a statement of 'if you bite me, I don't want you on me'. She can then go and blow off steam by playing by herself.
Likewise, your bird also has to be allowed to set boundaries with you. If they don't want to be petted, they'll let you know by moving away or making unhappy noises, or by nipping. Akimi's still nippy, especially if I do something with her that she doesn't like, but she doesn't nip hard enough to cause pain. I also don't force her to step up out of her cage if she doesn't want to come out. I'll present my finger and ask her if she wants to come out. If she doesn't move to step up, I'll leave the cage door open and sit nearby so she can decide to come out on her own. She also gets a treat when I put her away, and I don't usually have much trouble getting her back in her cage.
If she's perched somewhere I don't want her to be (mainly the back of the futon... she likes to climb around on the back of the frame, and I don't want her there in case she gets smushed), I'm more insistent about asking. I try not to chase her around with my finger so much, because that's annoying for everyone. If she steps up, she gets praised (and maybe a treat). If she really adamantly refuses, I just carefully pick her up like an ice cream cone (even if she bites) and put her back somewhere she's supposed to be. She hates when I pick her up like that, so she prefers to just choose to step up instead so she doesn't have to suffer the indignity.

Usually I just try to keep an eye on her when she's out and prevent her from going where I don't want her to go in the first place.
We actually had a rough start when I first got her. I was often frustrated with how grouchy she was with me, and I think she was likewise frustrated with me for trying to make her like and trust me when she wasn't ready to do either. One of the things that helped remind me to take my time and keep trying to work things out with her was reminding myself that I'd basically ripped her away from everything and everyone she ever loved and put her in an unfamiliar place with unfamiliar people, never to see her old home and owners again. Keeping that kind of empathy and compassion for how traumatic the experience was for her really helped me keep my patience in those early days.
Since we've been working together to set up mutual boundaries and learn how to communicate effectively, she's gotten much better. She's happy to play by herself on her cage or on a nearby stand, or to sit quietly with me (for short periods... she has a lot of energy and short attention span) and basically be a near-constant companion, rather than being a shrieky, bitey, nightmare velcro bird. She's still a little bit moody from time to time, but she's gentler and more affectionate than she was those first few weeks.

And on the off chance your GCC isn't just trying to figure out how things are supposed to work in his new home, it could just be that he's having a bad day. Birds are just like people in that way... they might have a super cheery and friendly personality most days, but they can still have bad days where they get grouchy and don't want to deal with anyone.
