If the gangs wanted to make a stand against the state, they could. They would need to take and defend their territory while increasing their numbers, likely via kidnapping and child recruitment.
That is a pure fantasy. You know how I know? Because gangs historically and worldwide occasionally tried. This fails all the time.
My original point was that there is utility in having an armed populace because it requires government agents to evaluate the risk of infringing actions more heavily. Even if the citizens lose, nobody wants to be a casualty in they're resistance.
It succeeds plenty. The list of successful coups by paramilitary groups is loooooong.
Hell, the Nazis started essentially as a gang.