I could see it in either tank or dps role.
in Tank role, I could see it playing like Lightning from FF13, in that it's a slick fast striking tank in that it has the fastest weapon enmity combo out of the 4 tanks, in that lets assume the three enmity weapon skills all have a additional effect while in tank stance called extra shot, this extra shot fires a bullet from the gun part of the weapon each time the weapon strikes. Its DPS Combo could be unique in that it works from any range, or the range of mid range to close since it's DPS Combo works as sword swings or single shots from the sword if not in melee range. So basically imagine Dark Knight with ninjas weapon skill speed after Huton is applied, but if you happened to for whatever reason not be in melee you can still keep the pressure on albeit with less enmity by attacking from range. That would be awesome.
All it's other skills could be the flash in the pan type stuff that fits the jobs flavor, if it's Magitek Knight/Soldier, then maybe it has tech skills that look like magic or something. If it's Judge as someone on a youtube video pointed out it could be called, maybe they have a mix of normal machinery tech and martial prowess with the gunblade.
as a DPS role, I could see the gunblader playing like Squall in FF8 or Dissidia, at melee ranged it could fire to add extra burst damage at the end of a combo string, and has several high end power moves that utilize the weapon a lot like how Squall's limit breaks worked. Mixed in with a few unique magitek spells, maybe this job is a hybrid of what Celes was in FF6 and Squall is in FF8, in that they are artificially imbued with the power to use unique spells gained from their association with primals.