Well, gang members go out looking for trouble, for one thing. For another, they would just as well use knives, bats, chains, hatchets, or any other terrible implement to commit crime. Banning guns just makes it difficult or impossible for non-gang members to defend themselves.
Gangs kill plenty of innocent bystander children in the crossfire. And gang members would be less effective at killing without guns. Which is a net win regardless of whether the victims chose a gang lifestyle or not.
How many people are packing and ready to draw in a moments notice? You better be quick because many crimes (gang or not) are ambush crimes.
Considering there are many thousands of gun homicides and only a few hundred justified gun killings, I think "guns prevent crime" is just a story people tell themselves. Gotta justify teaching toddlers to hide from active shooters somehow.
