The most realistic version of a gunblade would be a semi-automatic pistol with a curved blade from under the barrel to just under the grip. The latter would have to be extended about an inch to accommodate a guard for the blade without interfering with the magazine. Rule of cool would deviate from this, obviously, but that would be the basic idea. This puts it in a dex-based melee/ranged dps, oddly with hybrid piercing/slashing damage. Or SE could do something funky with crystals and make the 'gun' part a focus for spell slinging, and rely on enchanting the blade to give it more punch, which fits with an int-based melee/caster dps.