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While there may be a subset of gangs which also have Fascist ideologies, I don't see how one could claim all gangs express stuff like (among other definitions) palingenetic ultranationalism.
I believe most crime gangs fit that description.
On the other side groups who explicitly identify themselves as Fascist or Neo-Nazi etc. don't really differ from criminal gangs much. They have similar goals: Rule by the Boss, not restricted by any laws or elections.
Do you really believe that Britain under King Henry VIII and Italy under Mussolini were both Fascist?
I never found where my extreme dislike came from, now I know. They are organisations whose purpose is to destroy the individual by violence to propel a hierarchy with a single individual at the top. Even the fake conservatism is there, which can also be visible in Mexican drug cartels. A pox on them all.
A disturbingly large number of criminal organizations form on racial lines and hold explicit racial supremacist views. There's white supremacists, black supremacists[0], MS-13, and so on. Religious divisions are also fertile soil for crime; that's how you get all the Islamofascist terrorists[1] like Al-Qaeda, ISIS, Boko Haram, and so on. We don't usually think of any of these groups actually becoming the government, but it's entirely possible for this to happen (e.g. with the Taliban).
Going the other way you have police organizations like NYPD, LAPD, or LASD that are exercising sovereign power without oversight. They aren't explicitly racist, but are easily infiltrated by white supremacists who are part of gangs. In the case of the LASD, they have so many gangs in them there's a dedicated Wikipedia page for it[2].
It's easy to miss out on this because, for the most part, gangs are the Scary Other that your local white supremacist is using to make racial minorities look more threatening. That's because fascists have two kinds of enemies: their stated enemies - i.e. other races; and their de facto enemies - i.e. liberals who don't want a perpetual race war. Fascists will shout and scream about MS-13 or Islamists by day, while breaking bread with them by night. Because they are ultimately plotting the same thing: how to steal your money, your livelihood, your freedom, and your life.
[0] I have a friend who calls Louis Farrakhan "Blitler".
[1] Christofascist terrorism is not a matter of if, but when.
Depending on the political context, you could be describing anyone from tankies[0] to ancaps to Black Lives Matter. The only commonality between them is that they all used Twitter at one point, a microblogging platform with a hilariously toxic userbase for the amount of attention the media gave it.
I shouldn't have to say this, but being toxic doesn't make you a fascist. Furthermore, political disagreements tend to be self-perpetuating in ways that look like coordination from far away.
[0] Who will insist they aren't liberals, but America doesn't use that definition of "liberal"