Well, everything you need to know is in the general info. Anything else is up to you. Ive given everyone a lot of space to work with on their characters and backgrounds. You can just be creative if you want, but stay within the setting and timeline :). The stuff I have put up is only the important need to know stuffs.
A Gun Fighter is usually made, almost never born to be one. His/her reputation as a fighter using a gun is what makes that person known as a Gun Fighter, not just by that person carrying gun. They are basically normal people born into the Wasteland or within the Empire's territories, and how they go about to become Gun Fighters varies with what they do. A potential Gun Fighter must acquire a gun of his own first, the gun that will be his/her baby for the rest of his/her life, and he/she may simply set out into the Wasteland to start off their experiance, or stick around some place or town to make a name for themselves there. Basically, its sort of like a cowboy wild west sort of culture set in a near post apocalyptic sci fi setting. Famous Gun Fighters normally have a lot of experiances and adventures under his/her belt, and towns will know of the person from the stories that circulate in trade caravans and such.
Empire Gun Fighters are a different case. Candidates are selected to be trained into Gun Fighters by the army, and they are normally desensitized to make it easier to control them, puttig into a elite special group. Their reputation normally persists in rank or how well the public knows them.
If you were looking for how a person is born into Sanguine, you could say they could be born normally into a family living in a Wastlander town, village or city. Maybe they could be born in a chapel dedicated to the Death Cult, or maybe they could be born outside of civilization, inside the ruins where only scavengers and soldiers go.