Yeah, gublades have barrels alright, they're just virtually non-existent - which is understandable, considering that they're only used at point-blank range. The bullet is fired to the side of the blade, as Kimera 726 explained. This isn't so clear on Squall's Revolver, but is clearly visible on Seifer's Hyperion, which is basically a semi-automatic pistol with the blade attached. Look at his weapon when you Scan him, or else freeze-frame the intro FMV - you can see the construction of Seifer's weapon quite clearly near the end, when he slashes Squall.