I would definitely, absolutely never do the math like pastorlindhardt suggests above. Not only is any harm above 6 pointless, but this way ensures that the most valuable equipment is always the most damaged, and allows multiple "turns" of conflict to resolve at the same time. Harm rolls are in aggregate, not individually counted against another aggregate. It is also overly complicated.
It's far better to actually look at the stuff in the fiction. I would probably treat the gang as if it did not have vehicles point wise. Everything will average together. If they've got some special weapon like a real fully functional 50cal anti-tank weapon, maybe that weapon boosts the damage output of the gang by +1 harm while it's in play. But more likely, I'd do it like this: If you've got a missile launcher, all of a sudden maybe you can include that helicopter in the things taking damage during this exchange. If you've got a 50cal mounted gun, you can expect to deal your gangs harm against many or most or all of the vehicles in your opponents army. If you've got dangerous weaponry in your gang, you deal it against the other gang, and what could get wrecked by the type of fire is included in the harm roll. If you've got only light weaponry, then I probably wouldnt bother damaging any enemy vehicles, and the people most likely to be uninjuried in the exchange will be included as those manning those vehicles.
The reason any of this is relevant, is that having vehicles in an army allows you to make actions and do shit that you could not ordinarily do. You can run from a fight, possibly pulling out even your most wounded at high speeds. You might be able to engage an enemy easily, have access to further maneuvers and combat opportunities, you might be able to chase down fleeing enemy vehicles or guarantee that the enemy cannot escape. You could force the enemy to leave behind their wounded, or making hit and runs that let you whittle down a stronger foe. Having tactical and strategic assets in a combat never actually need to be explained or handled on a independent numeric standpoint (it can be on a case by case), but it is far better to let them determine the fictional possibilities of the outcome then try to figure out how to support the possibilities with the very simplified combat tools this system relies on. Fiction first, if someone's weaponry cannot harm a tank (sticks and stones for example), then they cannot roll to harm the tank anyway. Determine damage by considering what things are in play in the fight. Choose to take something if it makes sense, and be willing to take everything if that's what the fiction demands, etc.
Basically, you're doing it right. If you want it to matter more, then look at the times when they're doing stuff, take the time to make sure you comment on why it might be important to have... or to not have such equipment there. Explore that, push where they're weak, let them know where they're strong by providing them opportunities that their stuff allows.
Gangs don't use vehicle harm rules, that's for handling individual harm, not aggregate.