Allow me to point at Chicago for a moment:
They have many gangs, and the city wanted to stop them, so they enacted gun control.
But, because surrounding counties and states have lax gun laws, the criminals just bring them in. If everywhere had strict gun laws, then the only place they could get guns is from out of the country, but then they have borders and customs to deal with. Do you see how flawed your argument is now, or should I keep going?