I wouldn't see a machinist using a gunblade because they don't specialize in sword skills, but instead rely on mechanical turrets and regular guns. It just makes complete sense to have gunblade's within this game because it IS a unique weapon in the FF universe. And plus, this game needs a sword-wielding DPS job to begin with since we lack one. I am sure Square Enix could figure out the skill-set problems and create a unique sword-wielding DPS job by using the gunblade, balancing the ranged attack difference between the sword and gun, buffs/debuffs, and overall DPS to ensure it works right within the game. I don't expect the gunblade to make its appearance in the Heavensward Expansion, but for future patches it is possible, like what they did for Rogue/Ninja.