If someone pointed a firearm at a target, they should be fully prepared to fire upon said target. This renders any martial disarm imporbable.
Any offensive move from the target would be met by a round, generally aimed at the centre of mass. Shooting limbs is much harder and can lead to unwanted collateral damage as the round has to go somewhere behind the target. Note that different rules of engagements (ROE) do exist: LEO, special forces on missions, sicarios, … which will reflect what the shooter does from trying to prevent lose of life to "I was told to kill you, bye".
Martial art disarms are either play fighting or lucky breaks. In a few edge cases, where the ROE allow it, it is easier for a good martial artist to disarm/kill someone with a gun: We have all seen Dan Inosanto's video. Now, I would like to see the same video with Clint Eastwood (or any other fast draw chapion vs a noob in knife fighting: can a noob cover 21 feet in under .8 of a second? …
As an example of an actual fight of guns vs unarmed: Gates Of Fire ⚠ Graphic content Where a squad of marines get ambushed by some insurgents in Iraq. One of the marines ends up having to fight an armed insurgent without a firearm.
The marine wins due to luck and much better training.