G&G has been out for a while, but with a new adventure ahead for my group, I put some more though into the gunslinger. So my GM and I came up with a roughly workable design.
The basis is still the dueling pistol with repeating added, as "recommended" by the book. Then we cut the range increments down to 30ft and changed the handedness to 1+, meaning you need two hands to fire the weapon. The latter was to avoid dual-wielding, especially cheesing with Paired Shots in particular.
It is still not 100% in line with existing weapons, but much more reasonable, we think. If you wanted to really balance it, you should also make it advanced. We just honestly didn't care at that point, sorry XD
On a more out-there note, we also agreed that the gun could be single-loaded like a regular weapon, in addition to via the "magazine" of the repeating trait. It just feels good and right. If you want to properly balance it, you probably shouldn't do that either :D
If all else fails, the Rowan Rifle likely shows that you could probably also do this at a higher level as a specific magic item.
I hope this helps someone who also wants to play a gunslinger with such an iconic weapon ^^.