JoDizzle: I don't think it's true that gunslinger is an untenable play-style, you just need a saleable item that you can make.
TwoHandedSword: You're right in that there's a ton of ways to acquire GP in this game. But the issue isn't really money; it's the availability of bullets. A typical merchant stocks from 50-100 at a time, which can easily be used up in a few firefights, especially at low level. Running from merchant to merchant takes time; sleeping for a day (so that they can restock) takes time; running back and forth from dungeons to merchants takes time; scrounging enough of the components to make your own, even with the schematic, takes time. And carrying all those bullets can eat into your encumbrance, especially since as a gunslinger you're unlikely to invest highly in Strength.
The general store in Tarant carries 260 every day, so it takes a couple of minutes to rack up 1000 by waiting outside a day with the shop keeper offscreen. If you're using a one bullet per shot gun (which is most of them), that'll probably last you a fair few hours of game time. Regular guns are a piece of cake to be honest, so long as you have the cash. For earning money, explosive grenades at least will probably take you around 10 minutes to get the necessary tin cans and powder, and they make three and sell for 700 each at gun stores - so it's not very hard to rack up 10000 gold without breaking a sweat. On the encumbrance issue, it's better to use lighter armor - I tend towards featherweight chain if I can make it.
What I have noticed is that weapons that consume fuel instead are much more of a pain (e.g. Flamethrowers), as fuel is only sold in lots of 10 or 30 for a much higher price than bullets. If you get the buy the common schematic to make fuel, you can make 50 at a time with wine and yeast... but wine and yeast are only stocked about 50% of the time at a herbalist it seems (wine is more common at a general store, but yeast is way less common so I tend to stick with the herbalist). This means to get a stock of 1000, it'll probably take about 15 minutes.
Currently I've got Vollinger on a Flamethrower, Sebastian on a Grenade Launcher, and myself on Bronwyk's Gun - all requiring fuel (I was going for weird projectiles flying all over the place). To get us all up to 1000 ammo for the latter bits of the game probably took about three quarters of an hour. I'd say that's a pretty big flaw in the game, because to get potions for mages (if you even need the potions in the first place), it takes a minute or two tops. Same goes for other tech items, like the top tier ones in the Therapeutics discipline or Paralysis Grenades. But supply of bullets themselves, I don't have so much of an issue with.