The Gnomes don't have a superweapon. That's what makes them so powerful. From the (admittedly very limited) information we have on them, they're just using technology and winning off the back of that, which fits with the superiority of the Dwarves and the way that instituting meaningful reforms and using good tactics allowed Black to turn Praes from a laughingstock to a real power.
Gnomes absolutely do have superweapons. They glassed a town.
But you have absolutely misunderstood them if you think reforms or tactics have anything to do here. The Gnomes are as previously mentioned a plot device.
They are an explanation for the technological stagnation, thats it. Nothing more should be read into what they do, or how they do it.

It's more that Black is basically the only person in the setting who seems to really care about competent professionalism instead of story-powered grandstanding. He managed to conquer Callow basically by just "having a professional army" and "not behaving like the antagonist of a cheap novel". One of the big themes the series hammers on is the effectiveness of that kind of competence and training (particularly in war). If you look at the Tenth Crusade, basically the only reason the crusaders got anywhere against Black was Name cheating. In a straight fight, he would have held against them with close to 0 losses.
That runs contrary to the actual story. Its like you got an idea, and are now trying to force a square story though a round idea hole.
For a start, the entire Proccer civil war was fought without named on any side. That was pure tactics.
And Black got just as far by having a team of absurdly powerful named behind him. Warlock alone is a walking, army wrecking doomsday device.

While as for the 10th Crusade, its funny you call it name cheating. While completely neglegting the BS it is, that we got 1 side who is a regular medieval army.
And another side who better than real world grade explosives, as well as engineers who see in the dark...