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.
The bits of Hanno's POV we get seem to indicate that he's pulled so much in from past lives that he almost doesn't know which parts are him and which parts are old heroes anymore. Which is priming him for a hell of a fall at some point, though the exact shape that'll take is unclear as yet.
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.I would expect Tariq and Laurence to train the Heroes under their wings but very little learning of any sort seems to have stuck with Christophe if they did. I think Catherine's training under Amadeus must have been an unusual confluence of superb teacher and exceptional pupil as it doesn't seem to be replicated elsewhere.
Spoiler: New ChapterThat's true, but remember that Cat is on one side, and one of her signature moves is taking a Pattern of Three that's aligned against her to the face and winning anyway (Lone Swordsman and Heiress at First Liesse, Pilgrim at the Prince's Graveyard). I tentatively expect that if this does count as a Pattern, Cat will figure out some way to win regardless.