Speaking with experience of sword fighting (long sword), one potential use of a gun sword would be when sword fighting. In the event that you meet an opponent of equal skill, quite often you end up dualing for a prolonged period of time where you clash repeatedly, often locking blades and manoeuvring around each other to try to get lucky with the end of your blade when you separate (more than once I have actually dropped my sword on purpose here, whipped out a knife and stabbed the other guy while he was debating what to do with my sword).
At this point in time where the blades are locked, essentially neither party has any offensive or defensive ability. If you happened to have a pistol strapped to the side of your sword's hilt, letting off one shot at nearly point blank range would not need much aiming at all. Even if your shot only clipped your opponent's shoulder, that should be sufficient to distract them for long enough for you to extract your sword and run them through with it.
In a worst case scenario where the shot shatters your blade, your opponent will have just been showered with fragments of your blade, distracting them significantly while you still have a rather sharp shard, with a rather nice handle - enter the world of luxury shivs! Once your opponent has been dispatched, you can then continue with their sword and keep your pistol powered shiv for backup.
As most good long swords balance just in front of the crossguard, if the pistol were strapped to the hilt of the sword, possibly running across the crossguard, you could either reduce the size of the pommel or increase the length of the blade to compensate for the added weight of the pistol. As aiming is not really an issue, you just need a trigger you can pull/press with any free digit on your hand, so you only need to design a well made (read balanced) sword rather than a fancy pistol.
For the usage suggested here, the pistol does not actually need to be pointing directly down the blade. It could easily be mounted at any angle between matching the blade and being at a perfect right angle to it. I expect years of experimentation would discover the best angle. However, provided that the sword fighter was able to approximate a straight line from the barrel to the opponent at practically point blank range, this could be a very influential weapon for the first side to implement them. Very demoralising for an enemy to know that even their best sword fighters could suddenly be taken out by a comparative novice by simply blocking and shooting. No matter how hard you train, practising not being stabbed just after being shot is very hard to fit into a practical curriculum.
In the real world, once accuracy increased, dedicated guns took over from swords, making a sharp piece of metal a fall back weapon rather than a primary but in the era before that where guns were hopelessly inaccurate, being able to discharge one at point blank range while sword fighting could well have been a revolutionary tactic.
Unfortunately, by the time guns had been miniaturised sufficiently to be mounted on swords, they were reasonably accurate by themselves and could be used effectively as weapons in their own right or with a sword in one hand and a pistol in the other.
I think this could have been a very effective additional weapon for sword fighters had it been introduced when sword fighting was the primary means of combat. Reloading would probably have been an issue - very hard to put down your primary weapon in the middle of battle, sit down and do some work on it, so the pistol would have to be loaded before battle and used more as a reserve weapon than a main one.
An interesting alternate design could be a snap-on, disposable one shot pistol. That way you carry a pouch of these, whip one out and clip it onto the side of your sword after you expended the last one. If the mounting point is well balanced, the difference in weight between an empty pistol and a loaded pistol should have negligible effect on the sword's performance. This kind of re-loading action could be done with one hand, so given a brief lull in the fighting, you could reload mid-battle without risking sitting around defenceless for five minutes or so.
For a variation on the above, have the snap-on point as a standard fitting and allow other items to be mounted there too e.g. a pre-loaded crossbow, a spring mounted knife blade, a double barrel pistol, a bunch of flowers etc. anything that can distract the opponent long enough for you to let your sword do the dirty business.