Usually lore wise, gunblades are used as versatile weapons mixing meele and ranged, but lacking the specialization of a gun or a blade.

They are jack of all trades due to their versatility, so having them on a tank class would make them pointless.

Would fit on a dps class similar as how red mage mixes melee with ranged spells (imo should be more melee than chaining an empowered 3 hit combo then going back to ranged but thats another issue)