First off, historical dragoons were mounted infantry which is light by necessity; if they were heavily armored/armed, they'd be better served by remaining cavalry. Their purpose was rapid deployment and movement to which heavy armor and weapons don't contribute to. As such, the heavy armor that dragoons use in games, especially Kain, who used armor as heavy as Cecil's, aren't congruous with the historical units. Furthermore, dragoons in the FF series and in modern fantasy archetype, when linked to dragons, fought while mounted on their dragons as opposed to using dragons only for mobility, which was the purpose of the horses for historical dragoons. "Infantry" is the operant word in the term "mounted infantry", and it refers to fighting on foot even if the term "mounted" generally means "on horse"; for mounted infantry, "mounted" just refers to how they get to where they're going to so that they can start fighting on foot.
Secondly, the etymological origins of the term "dragoon", while not entirely concrete, are most likely based upon the French descriptor of the firearm carried by a specific evolution of their mounted infantry (i.e. the gun and dragons both belched fire), hence the etymological similarity of the two terms. Dragoons existed only after they began using firearms, before which they would simply have been mounted infantry.
Given how gunblades are used in game by the Garleans, I would expect it to be a DoW class that uses combo melee attacks as its baseline functionality with the non-combo attacks being ranged (think Dragoon but Heavy Thrust and Phlebotomize have a 20y range) in order to force melee to be the primary combat methodology. I'd expect to see some degree of magic-martial fusion as well; probably something like the blue fire and/or cross-slash that Gaius uses. They'd probably have slightly worse damage than the other mDPS, since they wouldn't be as screwed by short stints of being out of melee as the pure mDPS, but better than the ranged deeps.Now then, on topic, I'd very much like to see how SE would accomplish a Gunblade using class.