I have trained in multiple styles of Kung Fu, Some Karate, multiple Filipino styles based on knives and sticks, and a dash of Krav Maga. The only other style I've learned about is Libre System, and that's some great stand up fighting, but not used in relation to firearms. All I really learned of Krav Maga were disarms and some knife work.
In what I have seen and practiced, the full mount doesn't seem to happen as often as a lot of Jiujitsu stylists seem to portray. The video posted makes me think of two questions: Why draw while on the ground when you can wait for a better position? (It's showing you have a weapon while offering a dominant position to the opponent. Why fight over the gun when you could simply talk them down from here?) For civilians, why have your gun out against an unarmed opponent? If you're standing up, they are a threat, but not lethal, you should not be drawing. If you are young enough to use that technique, there's quite the chance you could use better open hand defense on your feet. If you aren't young enough, there's a chance that by the time you've drawn, been knocked down, and you are trying this technique, they may very well have beaten you.
On your feet, it's all in footwork, hands, and communication. But as we all know, excrement happens...
Carrying a gun that doesn't fire without a magazine is an optimum choice if you see yourself in this position. Drop the mag. Now you're fighting over a bludgeoning tool, not a firearm. If you carry a revolver, cover where the hammer strikes, or if possible, open the cylinder and keep it open. Secondary to these options, a knife is excellent in this position. Small and fixed bladed, icepick grip, edge in. Something like the SOCP Dagger is what I would suggest. This is of course my opinion according to my training and experience.
If you find yourself there, and you attempt this technique, and it works, that's great. I don't like that it involves aiming the gun in a direction you can't see, and that the opponent may very well rip the gun out of your hand before you can push them off of it, if you can. I would rather fight the structure of the wrists and hands by aiming the gun lower on them, making it harder to grip. Just a thought on it.
If you're looking for answers, it's sometimes best to learn things and try them out yourself. The best, most violent criminals do it, and it works for them.
"Today is victory over yourself of yesterday; tomorrow is your victory over lesser men." - Miyamoto Musashi
[Insert random irrelevant religious quote here]