...a 2mm slug with a battery attached intead of gunpowder....
See, and that is the reason, why it is not classified as a gunpowder weapon. Easy, isn't it?
It's the Fallout 2 classification which made not much sense, other than allocating some kind of weapon to the Small Guns category to have something to keep up with the other classes.
A Gauss Gun is an Energy Weapon, get over it. It actively propels a projectile by the use of induction coils.
That is something completely different to the hammer-hits-primer-causes-gunpowder-explosion-behind-the-bullet-thing, and you should know that.
You're just not willing to invest into your EW skill, and want some cheap way to get one of the best weapons of that class without doing anything for it, naughty boy :no: