All gun takeaways pre-suppose one thing i.e. the guy with the gun doesn't know what he is doing. It's not helpful to analyze gun takeaways from the standpoint of CQB trained firearms experts. They will not stand where you can touch them. If they had to stand where you could reach them, they would have the gun where you couldn't reach it.
Gun takeaways are strictly for an encounter with the amateur. If that gun is pointed at you and you aren't already shot, then there is another agenda; intimidation, hostage taking, something like that. Intimidation is the big one. If you've ever had a gun stuck in your face or in your belly you know that it works just fine... it's intimidating as hell.
If the guy is dumb enough to touch you, then takeaways are fairly easy. My objection to most retention or takeaway systems is that they do not utilize enough impact technique. If you take a gun away from someone and you haven't struck them, you are almost certainly now grappling for that weapon.
All of the best retention and takeaway systems I have seen involve serious impact and preferably balance breaks thereby giving you the time to bring the firearm to bear on the assailant. Aikido - Aiki jutsu derived techniques are great for this but you need to have major atemi and you should train to shoot the assailant as part of the takeaway. Then, in a real situation you can decide not to shoot but your default setting is to take the gun and use it on the subject in as close to one smooth movement as possible.
Most retention and takeaway training is done with a partner who does not have strong intention. It makes the whole thing appear too easy.