He brings the ball down to low where guards can strip him, and then he tries to dunk it every time and big men will block it. Greg should ask Joel to get some pointers, since Joel's shooting 85 percent and gets a lot of his in the same position that Greg finds himself in.
He does a couple of things that should be completely correctable with practice and experience: 1) he brings the ball low when he gathers himself instead of holding the ball right at his chest or under his chin which makes it easier to strip, and 2) he's still probably recovering his explosiveness after MF surgery and as he improves he shouldn't have to spend so much time getting ready to explode.