Gun defense is based on being close enough to the attacker that you can grab the gun. If you have to step, run, walk, fly, or apparate to reach the weapon, you are back to bullet defense, which is not considered a good strategy.
As a child my brother and I played a game where we tried to slap each other's hands. There are many variation on this theme, the most popular has the 'attacker' holding their hands out in front of them, about waist high, with the palms up. The victim, places their hands palm down on the attacker's palm up hands. The goal is for the attacker to quickly flip their hands over and slap the back of the victims hands before the victim can move away.
The movements of the gun defense must be of the same mental nature. Fully committed, with no forewarning. Again, there are many approaches and techniques but they all have the mental characteristics I've described here.
The last element to understand is that bullets travel in straight lines. They do not curve. Therefore, you must move the part of your body that is in peril out of the way. But, but, but ... and this is critical, in the time frame we are talking about, your feet are snails in a land of Road Runners. You cannot move your feet! Moving your feet is a bad strategy.
Here is a video for a gun defense using a technique taught in Krav Maga. Note that the defender does not move his feet until he has control of the weapon and his head, the target, is out of the line of fire.