Twist the gun to face downwards, which can break the person's finger. Then, point the gun at them.
If the person is being me, aiming for my back or head with a gun, I could try whipping my body and head around to face the person, stepping under their arm and snatching the gun. I would point the gun at them after all that
After getting the gun, I'd slowly shuffle back and, if I'm far enough away, run for it.