The biggest gangs in the USA tally in between 20,000 and 35,000 members. (Those are major gangs like the Bloods, the Crips, the 18th Street Gang, and others.) They are actual criminal enterprises with serious membership and, in some cases, run the gamut from interstate to international.
You’re more likely thinking of a lower level street gang with maybe 20-30 or up to 100 members. I would run with that because it’s much easier to manage than the massive “we can take over prisons” gangs. If not, then he’s probably a local leader or a chapter head working for a much bigger fish. Even a lot of smaller gangs work like this once they go into criminal enterprise where they have possession of a certain territory within a city, but keep that territory by owing someone else up the food chain who funnels in the drugs and other goods they traffick (if they traffick). They may keep their name while owing their allegiance to that other crime boss, who in turn leaves them alone so long as they flip a profit. This can run from smaller gangs to bigger gangs up to Colombian drug cartels.
If you’re going to put together a gang, it’s worth asking yourself what kind of gang he’s put together. If we assume that he’s into something more serious than a bunch of kids who spend their nights tagging the sides of buildings, then you’ve got to start asking the real questions about criminal enterprise that any kind of business has to answer.
How do they make their money?
Where is the money? Where does it go? Who does it go to? Who collects it? Are they running a protection racket? Are they into gambling? Are they smuggling? Are they trafficking in illegal goods (drugs, stolen cars, prostitution, etc)? How do they pay for their weapons? Do they counterfeit? Go into indentity theft? How do they buy their bullets? Gas costs money. Where does the money to pay for their activities come from?
There are street gangs that work on the level of organized crime like the Mafia or the Irish Mob and the line of what makes a gang and what doesn’t starts to blur.
It’s probably worth mentioning that the modern Hell’s Kitchen has been a victim of gentrification. You’re going to find high priced townhomes, apartments, condos in some very nice neighborhoods. It’s not the home of the uber rich, but it is a very nice place for the upper middle class to live. You’re more likely to see it targeted by white collar criminals than your average mugger and biker gang. This doesn’t mean that they’re immune from crime, after all your gang members could be targeting the lonely kids living there through sites like Facebook for recruiting and access to their parent’s credit cards/personal information.
Remember, just because the kids don’t live in impoverished neighborhoods doesn’t mean they are incapable of joining gangs. In the age of the internet, new avenues have opened that weren’t available before. The kids who come from rich families are often just as lonely and therefore just as vulnerable (if not more so due to the distance) to what a gang offers than a kid who grew up next door to it.
Gangs in the US, Wikipedia in general should be looked at askance as a resource, but it’s a good jumping off point. Many of the links listed here as source material will link you to better and more reliable sources where you can begin your research in earnest. It’s a troubling subject, to say the least, but good luck.
Monster by Sanyika Shakur, aka Monster Kody Scott. This is the autobiography of Sanyika Shakur about his experiences growing up in South Central L.A. and joining the Crips. If you want to write about gangs, I suggest giving it a read.