I'm pretty sure it was not a shockwave, but an actual projectile, but since there is no barrel, the projectile has a restricted range that's only just past the end of the swordblade, so it can't be used as a regular firearm. I think the gunblade was retconned to fire actual distance projectiles though. At least that's the impression I get from Dissidia. Didn't Weiss from DoC wield dual gunblades?

I've always thought of gunblades as essentially being the reverse of a rifle and bayonet. Whereas a bayonet serves no real melee function on a rifle other than a quick stab, the gun part of serves no greater purpose than to enhance the damage potential in the slash of a sword.