The subject of gunpowder weapons in fantasy RPGs leads to a lot of debate. In the first ever game I played in, gunpowder didn't work because the gods didn't like it and simply caused it not to work--except for the god of the sea who loved the noise. So, gunpowder only worked at sea. Pirates, HO!
Anyway, I 'm okay with gunpowder in fantasy games, but not excited about it. I don't want to see castles and plate armor made obsolete. So first off I make gunpowder an expensive alchemical magical product. Making gunpowder is not simple chemistry with ordinary materials, it requires arcane magical processes. Thus gunpowder is not only relatively expensive but only available through certain sources. In my current game all gunpowder has to be imported from far off foreign lands, making supplies uncertain. And since gunpowder weapons are fairly rare, proper gunsmiths are also rare. Your typical blacksmith can usually copy an example but if working from an inexpert oral description the resulting product may be prone to bursting.
I also like to make firearms primitive, like they were in the late 1400s and early 1500s. It's handgonnes rather than flintlock muskets. More like in this European medieval re-enactor video or in the early part of this Japanese re-enactor video.
For Old School Hack, my current game design obsession, I would make them Heavy weapons. It gives them more damage than an ordinary weapon but makes them clunky to carry and use.