The main stats a gunslinger needs are dexterity and perception, plus enough strength to meet the minimum strength requirement of their weapon. You want dexterity at max as quickly as possible in your character's life, and perception probably at max, but 18 will suffice.
I wouldn't bother putting points in gunsmithy for any reason barring roleplay, because it doesn't give you anything that you can't acquire from various NPCs through the game anyway (albeit in limited quantities). More generally useful tech disciplines to invest in are herbology for curatives, electrics 2 for a pair of charged rings (that's +4 dexterity assuming you have something in the order of 16 or higher tech aptitude), and explosives 4/chemistry 1 for stun grenades, which are very good (both for fighting and selling en masse for easy cash), as well as paralysis grenades, which are obscenely overpowered and work against everything in the game, even mechanical enemies and undead.
If you take Magnus and get him a few levels, he can make you a set of eye gear, which will add +2 to your perception. You can use this along with a helmet of vision to build a goggled helmet that gives +3 to perception, but you need smithy 5 to build a helmet of vision, and I don't think any can be found in the game otherwise.
You get a fate point from dealing with the brigands blocking the way out of Shrouded Hills. Save it until you get to Tarant, then find Sammy White and use it to steal his hand cannon. It's the firearm you get from investing in gunsmithy 6, it does a lot of damage for the point at which you can acquire it, and it's the best pistol you can get until very late in the game.
Also keep an eye out for elephant guns (you can acquire two in the game from NPCs), and looking glass rifles (you get only get one of these without gunsmithy). Elephant guns are a powerful weapon on their own, and can be used to create a blade launcher with a schematic randomly stocked by quality smiths. Looking glass rifles are middling weapons post-patch, but are a component for creating tesla guns; inventors' shops randomly stock the schematic for those. You need electrics 7 to build a component for the tesla gun, however.
Some other assorted notes that might be useful for a gunslinger:
- Carrying a lantern or electric light in your offhand *does* increase your accuracy at a range in darkened areas, if you didn't already know this.
- All firearms are obviously tech weapons, and so if you're fighting enemies with magickal aptitude, your to hit chance will be penalised if your tech aptitude is lower. The game likes to be a dick with this by flatout giving enemies 100 MA, especially things like ore golems. Try to keep your tech aptitude high.
- Sniping actually does work well in this game in turn based combat, if you have a firearm with a long enough range to be able to hit enemies and still hit R to end combat (and that's not as long a range as you might imagine). It's possible to shoot your enemy until your AP runs low, then tap R twice to fully replenish it without giving them a chance to advance on you. Repeat until they're down. This is cheaty, though, I suppose.
- If you're at a loss for things to spend character points on, it's always worth it to invest in willpower up to 12, and then buy 3 points in temporal magick for the hasten spell. It's easy to maintain on its own, it ignores your magickal aptitude or lack thereof, and it doubles your speed, which is a tremendous advantage for a gunman. There are a few magickal colleges like this for which your magickal aptitude is largely irrelevent, but temporal really works very well for anyone. Techno-mages can be quite powerful. If you don't feel like dabbling in magick, though, you can also just stockpile potions of haste from gypsies; these will work regardless of your tech aptitude too.
Good luck!