this works also
"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..."
This can work even without a knife. If you make your hand open, fingers togethor, and stiff as you can get it, then place the pinky under the nose and against the teeth then press up and into the skull as hard as you freakin can the tendancy is for the head to go that direction and the body to follow. This has to be practiced to be effective and needs to happen very fast - remember the BG is in a fight for his life as well and will try to bite, scratch, kick... etc just as you are doing.
This is something that if practiced can be followed with a strike to the fleshy area directly behind the chin and more then likely get you far enough away from the BG that you can then either run or do whatever else you have to do based on situation.
Note that you can get creative with this also, as another person suggested, poke them in the eyes. Most people I know will drop fast when there eyes have been comprimised...
There are many targets available, the biggest thing is to train on how to use them.
If you train your much more likely to succeed when you need it...