There's two main reasons people join gangs, and that's survival or respect.
The Guristas offer both these options. To the citizens that can't make it in a meritocracy, gangs are the best remaining option for income, food, shelter and protection. While you could join any insular gang to get this same effect, the Guristas are the biggest, toughest, meanest, and best equipped - and, to my knowledge, the only guri organization with access to spacefaring vessels. Could be wrong though.
That covers survival. The other half would be the percentage of citizens present in every society that, due to factors of environment, upbringing, neighborhood, and if you subscribe to the belief, genetics, cannot function normally. Some children may be raised in guri neighborhoods, or by a guri brother, or even parents. Some individuals might simply be averse to normal society and seek out the underground. Some might be brought in by friends after living in the club scene for too long. Whatever the reason, some people would see gangs as a means to get the respect other citizens get from serving the State and bettering themselves.
Rebelling against the State was Fatal and Rabbit's goal. As the organization grew, however, I imagine that simply remained the leader's goal. The hoard of soldiers, probably owe their membership to one of the above two origins.