Or play as GLA/PLD and glamour the Blazefire Sabre from the FFXIII crossover event onto your sword.

Really, I don't see this being ever an actual class - it should also be noted the Garlean gunblades are nothing like Squall's one (the later being a sword with a rumble pack attached, the 'gun' in the title is a misnomer), Garlean gunblades are in fact just a regular firearm with a blade attached to the side. And as they only seem to be carried by elite Garlean troops like Centurions and Legatee like Gaius, it's likely they;re in fact a very rare weapon type, making a gunblade using class even more unlikely.