The Gunblade "class" has been around for quite a while for Garlean NPCs, most notably Nero before switching to his hammer in the Omega quests and now a new character introduced in what is more or less the outro for 4.3 MSQ. And to some extent Regula from HW, but his's sword being a 2 hander had slightly different animations...

And for roles, they could always go back to having one of each role... If it does indeed involve Garlemald, the gunblade one could easily be the DPS class, for healer possibly Chemist. Since the Garleans aren't exactly able to use magic as stated in SB, they would probably have other means of healing.
For a tank, I dunno from Garlemald, but since Ivalice is a thing, maybe takee the Gladiator class from Tactics Advance and A2, rename it and combine it with Mystic Knight and make it a tank.