Gangs. Mmmmm let me think about this. You mean the mindless doped up drunk idiots that run around shooting people at random type people ? They are just that, only lots of them. Will they be a threat to us ? Only for a short time and for the most part mostly in the cities. Out here in the woods where I live, it gets dark at night, wild animals such as cows and chickens etc, would all lead up to them making a long trip for nothing. IF you are one of the less fortunate ones living in the cities, the chances of you surviving long term all almost nill anyway. You will have no way to grow your own food, obtain water and litterly trapped in your environment. IF you really want to survive, start planing to move to the country and do it now.
Look at it this way. There you are in the city and you are happy as a clam, for you have managed to create a really productive container garden in your tiny little back yard. As soon as you start cooking that harvest, people will smell it. Even it it isn't the "gangs" your talking about, there will be plenty of mob's running around searching for food. You cook it, they will come. Gunfights, where, at your front door ? How long will that last do you think ? About 5 minutes before the blood gets slippery and the flies land. I hate to pop your bubble, but in reality, you don't win a gun fight. At least not a sustained one.
To survive a major everlasting event, you need to not be in the city. Even getting out of there will be a life or death situation. So, here you are already worried about the gangs, which most likely means that they are already a major concern of yours, knowing full well that given the chance they will only get worse, and still you sit there and do nothing. Your looking for a fast solution to a very bad problem. There is none. Bottom line, again IF your in the city, you will perish at some point and time before everything even begins to get some kind of normalcy once again.
The best solution of course is to remove yourself from the city and do it asap. Sure you will make less money out here in the rural areas. In fact it can be darn near impossible to make much at all. But, I can grow as much as we can eat, trade with neighbors for my crops for theirs , chickens, cows, goats, and even fruit somewhat. My solar is all in, wind, back up everything for my solar, 3 gennies for even more backup. Will the 'gangs' make it this far ? Not likely at all. I made this choice about 16 years ago, and have never looked back. My chances are far better then anyone living in the city. Do I have lots of guns and ammo ? Not really. Do I see myself having to worry about gangs ? No. A few really desperate people ? Most likely some anyway. What my lifestyle realy does do, is it allows me to sleep really really well at night. Quiet, secure etc. My dogs , are first line of defense, "driveway alerts" second line and even some in the house in stratigic places. I have food and seeds put away, and know how to garden. All you combat heros in the city should give this some real thought. Oh, by the way, yes it is hard to do, especially at first, but after a year or two , it begins to get much easier all the time. Good Luck