The main problem with ancap is that there's nothing preventing a violent gang (a new government if you will) from forming, because everyone else is being so peaceful, and if everyone else stops being peaceful, then it it isn't ancap any more - its just regular anarchy.
Ancap will only work if there is some force to stop people from conducting acts of violence, preferably some force that isn't corruptible. So, until we have open-source peacemaking enforcement droids no such utopia can exist.
That was the case many years ago when the nobility had castles, horses, swords and so forth and the peasants had pitchforks and sack cloths. And true to a degree currently where the government gets the most fun toys. But in a modern society, the people would have access to the same kinds of weapons as your gangs and the gangs would have a much harder time establishing any kind of footing. It's no accident that the American revolution occurred at a time when important changes in personal armaments were happening.
And after all, it's not like places with governments are exactly gang-free either. Especially where the citizenry are disarmed.
Exactly, thats the point. People do have the same kinds of weapons as your gangs, so everywhere a gang would form, a counter-gang would form, and the winning gang would surely be angry at the losing gang (especially if fatalities occurred), and the losing gang would flee to a surrounding area (or get killed off). Once they flee to a surrounding area, they will pick what they think is the winning gang there, and try to join it, resulting in each area being run by 1 gang constantly in fear of being taken over by its counter-gang (who they will naturally call "criminals") and surrounding gangs who have allied with "criminals."
And then the winning gangs become governments and we're back to square 1. The problem here is human nature: Humans like to form teams, and the winning team tends to punish the losing team, who then wishes to get revenge.
Decentralized violence begets more violence, and violence begets centralization. The only way of avoiding centralization is either reducing it just enough so that no violence breaks out, but government is still sufficiently small, or by somehow getting rid of it entirely and replacing it with a system to prevent violence hands down.