Demacia's got stuff made of Demacian Steel, so they don't really give much of a toss about guns.
The Freljord's far removed from modern Runeterra, so they don't have guns.
Noxus is modular in how it arms it's soldiers in that you can use what you want, so they have guns on an individual soldier basis.
Ionia's not really been about traditional guns, focusing more on bows and arrows, but with hextech creeping its way in to their culture traditional guns might be in the cards.
Shurima's in the same kind of situation as the Freljord, but a bit better off on account of Piltover/Zaun being nearby and Noxus having influence there.
Piltover, Zaun and Bilgewater are fine for guns.
Can't speak to any of the smaller, independent places of Runeterra like Krexor, Stonewall and Holdrum.