So a couple of things to consider with the setting and real world terms. It doesn't matter. You're writing a game and trying to get across an idea as best you can and as quickly as you can. Sometimes ten words fits better than one hundred. And sometimes its just easier to use a real world term in a fantasy setting because its something that is immediately understood.
If we have to justify the use of the term "gunboat" (not that there should be, its the writer's prerogative to decide what they use), we have three ideas that can easily work and keep in line with the Bronze Age setting of CoC2.
1 - Ballistas and Onagers, or as we call them nowadays - bolt throwers. Precursors to cannons in that not only could they fire bolts, but also spherical stones. Perfect for facing off against masses of troops or besieging a fortification. Mount them on a ship and you have something that can besiege the enemy from the siege. Or if the enemy is using something like a trireme, a bolt could be devastating against the rowers.
2 - Catapults, mangonels and depending on the time period, trebuchets. Some advantages over the ballistas is that you can throw more things further, ranging from larger stones, pots of Greek Fire, dead cattle...although if you're on a ship, you could replace that with rotting sharks, chunks of festering whale blubber and so on.
3 - Mages. CoC2 is a setting that has magic. You could easily imagine a number of mages serving as a battery of arcane firepower.
Bear in mind as well, that the world of Savarra is still recovering 200 plus years after the Godswar, the Belharan Empire is just a shadow of its former self, not to mention that a great deal of knowledge has been lost, both to the Godswar and the collapse of the Belharan Empire. And that could have meant cannons and the like.