Tactically...
... kid initially goes for both keeping the gun directed away from himself, and for taking the fat gangsta's balance. This is good. Kid doesn't really seem to focus on directing the gun in any particular direction, but then again there are people in most directions and that would have taken a whole different level of awareness and capability. This is not good or bad, it just is. Kid's focus seems to go away from gun almost immediately after takedown. This is bad, if he doesn't KNOW the gun is currently out of play and out of reach.
Not sure where gun ends up. Other skaters may grab it. Otherwise, there's an unsecured firearm that everybody is ignoring, despite the yells of "he's got a gun."
As somebody else noted, during all the time kicking and punching, nobody really disables gangsta's hands or ties up his arms. If he had any other weapons, this could have bitten them all really badly.
Kid recording video would have been smarter to take a moment to secure the weapon, but didn't seem too worried about that. Kid recording video didn't yell, "GUN!" when he saw it, and his actions seemed more like those of somebody thinking, "cool, check this out!" than "oh sh**!!" (IE filming, maneuvering for better angles, not helping his buddy, not warning others)
As somebody else noted, skateboards make more effective weapons if turned like a blade, to increase surface pressure. Same would be true for an oar or paddle, or any hard, flat object.
As I noted, I wouldn't recommend posting a vid that is guaranteed to rile up a gang. I wouldn't recommend posting a vid of a bunch of my buddies kicking a guy or hitting him with objects, as that might rile up cops and DAs. I would recommend reporting any incident of having been threatened with a firearm to the police.
Guard should not have walked up into the middle of a crowd so casually, and by himself. Guard should definitely have called for backup, right after he called 911 or Canadian equivalent (guy who posted video had Canadian flag by his handle; hard to tell from accents and buildings where this was).
So, there's my discussion of the tactics in this event that for any number of reasons seems staged.