I've used various different firearms using classes in my current campaign and probably my favourite to date is the Arquebusier from kobold quarterly issue 13.
My biggest criticism of the gunslinger are the proposed rules for firearms. The reload time is unrealistically fast and the damage too low. With firearms being prone to misfire, why would anyone in a fantasy setting prefer these to, for example, more reliable and equally damaging longbows or crossbows? OK, the critical damage is higher, but still the expense and range of dfirearms make them a poor alternative to bow weapons.
A napoleonic musketeer would perhaps get 3, or 4 rounds off a minute if an expert. So that would be a reload time of three rounds, two rounds with rapid reload. Reloading in 6 seconds is ridiculously and unbelievably fast. I would increase the reload time for firearms, but also increase the damage. We use 2d6 for a wheelock horse pistol or flintlock dueling pistol, 2d4 for a snall snaplock, 2d8 for a snaplock or wheelock musket and 2d10 for a 'brown bess' musket. what tends to happen is that, as in history, players tend to have multiple pistols, and after an initial, high damage flurry, either drop the weapons and use a rapier or similar, or duck for cover and spend multiple rounds reloading. My players like this as it makes fireams more historically realistic and also makes them substantially different from bow weapons.
Weather should also be a consideration for firearms users. The best way to make blackpowder weapons useless is to throw a thunderstorm at the party.
These comments are based on three years of playtesting with different firearms rules with three different groups (including some ex-military players who have used historical blackpowder weapons).
The biggest differences between the Kobold quarterly Arquebusier and the gunslinger are the 'grit' rules, which seem to be a combination of a ki pool and action points, and the various crafting abilities and 'double tap' powers of the Arquebusier. It seems strange to me that a swashbuckling action points system is inserted into a firearms using class. It would perhaps be better to have a swashbuckler class, with an action points system for high drama "million in one chance" abilities that is more open (ypu could have dread pirate type characters, or Indian Jones type characters, not just gunslingers), and perhaps have a 'musketeer' or gunslinger prestige class to emphasise using firearms in the system.
I personally like the crafting abilities of the Arquebusier that allow weapon and black powder crafting that is lacking in the gunslinger. How often will village stores sell bullets and gunpowder in a campaign world? Unless firearms are ubiquitous, your gunslinger is going to find it hard to find ammunition. Also how frequent will magical firearms be? Mages will have been constructing magic swords for thousands of years, but presumably only magical firearms for a century or so, so numbers would be expected to be low. The powers of the Arquebusier give firearms users more equality in the magic items stakes.
Finally, having two free pistols (2000gp) of goods seems over balanced, unless the character has constructed them themselves. The starting cash of the gunslinger should be much lower to balance out their free high cost weapons.
I would give gunslingers a bonus with craft firearms and craft alchemy from first level, and only one weapon. I would remove the 'grit' system and instead place it into a more developed and less specific/specialized swashbuckler class, and give the gunslinger powers, like the Arquebusier, for magical ammunition use and focus on, but modify, the targetting skills in the gunslinger (but remove teh grit point system).
