If you're talking about a self-defense situation, if you are grappling with your gun out, it might mean that you should have long before that point shot the guy: if you have let him get into arm's length and given him the opportunity to put his hands on you, you have made a mistake somewhere, to be sure.
If you have not yet revealed or drawn your gun, and you have begun to grapple, it seems to me that you had better pour on the steam and kick his butt at the fisticuffs, because drawing your gun while attempting to control him with your single remaining hand will probably end up meaning that he takes control of your gun arm with two of his own hands. You need time and distance between you and him before you re-attempt to introduce a gun into that particular situation, yes?
-azurefly