Gunner. Like FF X-2 Yuna Gunner, dual pistols, focused purely on Gun Kata, sure we have Machinist for a gun-wielding job, but MCH is too much of an engineer job that happens to use a gun sometimes, and if we can have 6 different Jobs that wield some variety of sword(PLD, DRK, GNB, SAM, VPR, RDM), having different varieties of guns should be fine.