The primary advice for when someone attempt to snag your gun from it's holster is hang on!
If you carry in an open-top holster with no thumb-break, safety strap or other retention device, you may be looking at your own gun before you can act. Whether your holster has a retention device or not, get your hand on your gun and/or on HIS hand and push DOWN to keep the gun in the holster. Meanwhile, your other hand is trying to rip his eyes out, strike the adam's apple or otherwise inflict pain or damage while you move into him with your shoulder and drive him against a solid object (wall, post, car) as hard as possible. Twist your body to get the gun outta his hand.
Someone going for your gun is an immediate life-threatening situation. Bite, claw, scream, spit, kick, gouge and generally fight dirty to get him to let go and so you can get away.
Once you get clear, move away, draw and hold the gun with your forearm tight against your side pointed at him.
BillCA in CA (Unfortunately)