Hey! Welcome back! So, I see four options. The first is, most folks just ignore the gun category in fantasy games. At least, that has been the case in all of my fantasy games using this ruleset, either as a player or DM.
However, the second option is that you might just use it as is. There is a class, the Hunter, that might make use of a projectile weapon. Or imagine a Bard duelist who uses a pistol instead. Or somewhere in your world, on the DM side, there’s a goblin or dwarf with a homemade blunderbuss that needs to do a category of effort higher than a D6, just to distinguish that rare weapon. Or maybe you want to run a pirate game, or a three musketeers game, and in that case, you’re all set.
A third option is to do what you have done: carve out a category of weapons in your mind that deserves a dice category higher than D6. I love the idea of having a Zwiehander or other large weapon potentially doing more damage. In this case, though, we don’t always get hung up on the name or try to re-write the rules with a bunch of formal lists of stuff. We just simplify say, “oh, that massive Zwiehander you found, that does ‘gun’ effort,” or “that does d8 effort instead of d6,” and then we all just move on. I don’t think there is a need to overthink it beyond that, unless you just want to; otherwise, I see a lot of folks spinning their wheels and needlessly overcomplicating it. I also tend to prefer this approach rather than formalizing a list, because you’ll make a player feel super unique and special when you say, “oh, but YOUR weapon does d8 damage instead of d6.” In other words, keep things a bit more loose and a bit more rare.
A fourth option is that you save that d8 category as a milestone for your players during the course of your campaign. At about the mid-way point, the characters start finding d8 level weapons, graduating from d6 weapons, as the threats get tougher. These weapons are all “masterwork weapons,” or some label. And then later, they can graduate to magic weapons and all do d10 damage. And then they can level up to legendary weapons and all do d12 damage at the end of the campaign. In this fashion, you can bake in a bit of a Diablo or Borderlands tiered loot system.
In any event, I still recommend keeping things a bit loose, just to give yourself some wiggle room, whereas I think the moment you start defining lists of weapons that fit different categories, then you have to stick to those lists, and you’re just giving yourself one more thing to keep track of.