Pretty much what Bill said.
If you have a knife off-side, deploy it if you can (I'm guilty of not practicing this enough). Get that blade right under the grabbers nose and PUSH UP HARD!!! He will most likely let go of your gun...
If you use a knife on his grabbing hand, do yourself a favor and don't slash, you're just as likely to cut yourself. Put the point on his wrist (or wherever you have a clear opening), and shove it in as hard as you can.
If you don't have an off-side knife or BUG, stomp, bite poke, knee, etc....
Don't play fair is the bottom line.
My name is Steve, and I'm a Glockaholic.
The primary target is what we call the thoracic triangle. Shots to this area tend to make people FDGB (“fall down go boom”).