One of my cats was an excessive groomer as well, but just licking, not biting. Her whole tummy and the insides of her hind legs were bald. It started when she was about 7 or 8 and she did it all her life. I think it was stress that originally caused it, but as catloverami said, after a while, it just becomes a habit, and it's virtually impossible to stop. It can also be brought on by boredom, though it doesn't sound like it in Gizmo's case, since it seems like you've had him since he was young (and presumably energetic and not ever bored!).
But the fact that he's biting makes it seem like something's itchy. If you haven't already, I'd do as zuma suggested and have him tested for allergies. It might be something as simple as changing a cleaning product that you're using; some kitties are very sensitive to chemicals. My other cat started pulling out all of her fur at the base of her tail at one point, and it turned out she was allergic to the carpet freshener that I was sprinkling on the carpet (ironically, to get rid of pet odors), and there's another member whose kitty was allergic to the same product.
I hope you figure out what's causing it and that you can find a way to break him of this habit!