The problem with being a city is it’s still subject to the laws of the enclosing political entities. Unless you can get it to be fairly autonomous, or perhaps set up as a social experiment by the larger nation, you can’t change the war’s own interaction with the problem.
In particular, you need to decriminalize the various substances that keep the gangs in power. Why do shady deals with a gangster when you can go to a proper store that’s accountable and provides legal recourse to customers who are cheated etc.?
Everything that fuels the black economy and gang power base should be dragged into the light, regulated, and taxed.
A recent show I saw that includes references and citations(!) mentioned a study comparing well-matched populations of black vs hispanic incarceration rates and gang activity, which concludes that the difference is the presence or absence of a strong father figure. Using this as an example, provide family support and youth programs. Find various studies that credibly show root causes or necessary criteria, and address those. Note that this means really addressing the causes, not getting the program watered down or politicked into some near-miss or ignored due to religious reasons.
A good story would be to turn everything on its head. Find every way that current policies and programs are causing the problems, and do the opposite.