Now you've crossed over into the silly. I never said that gangbangers (or anyone) MUST be allowed to carry any firearm, anywhere, nor do I think so.
First off, a high-percentage of 'gang bangers' are prohibited from possessing a firearm, due to criminal history. Felons have given up their voting rights, and their Second Amendment rights. So if your average gang banger shows up somewhere and starts waiving his Hi-Point 9mm around, he's (probably) committed a crime before he even clears leather (or waistband) with it - felon in possession of a firearm. Go directly to jail. Do not pass Go. Do not collect $200.
Second, a person's or business' private property rights trumps one's right to carry, IMO. If an establishment has a 'no firearms' policy, then I don't go there, or if I must, my firearm is safely secured elsewhere. IMO, the Second Amendment is to keep the government from taking the citizenry's guns, and as such, has no bearing on a private party's on-premises prohibition of guns.