Welcome! First, a caveat: firearms are surprisingly weak, until you can find/buy/build the best weapons near the endgame. Bullets are expensive; and while you can mitigate that somewhat by making them yourself, gathering the components is akin to grinding.

To be charismatic, you need Persuasion, which has Charisma (CH) as its prereq. You'll want 20 CH eventually, both because it will allow you to choose and keep any available follower (regardless of alignment) but because the Persuasion master quest is the only one to require a full 20 (rather than the usual 18) for complete success.

To be a gunslinger, you'll need Firearms, which has Perception (PE) as its prereq. To build firearms (not required, but the only way to get some of the better guns) you'll need the Gun Smithy college, which has Intelligence (IN) as its prereq.

Dexterity (DX) allows you to fire more often. To save on points, you can let slide Strength (use followers as pack mules), Beauty (it only affects first impressions), Constitution (buy or make Fatigue restorers), and Willpower (unless you also want to become a master Haggler, which kind of ties in with the theme of this build).

Also, spend two points into either Melee or Throwing early on, so you're not skunked should the bullets run out. Throwing also helps with grenades, should you decide to invest a few points in that college.

Put one point into Explosives, and buy the Bullets schematic ASAP. As a bonus, you'll be able to make Molotov cocktails, which are far better than guns in the early game. (This is also where Throwing will help.)

If you want an early tech bonus, consider starting as a Dwarf. Definitely don't be an Elf or Half-Elf. Any background that doesn't penalize PE, CH, or IN (or give you a reaction penalty) is fine; you can also go with no special background whatsoever. (Miracle Operation is an excellent choice, since it gives you big bonuses to all three of those stats.)

There are people in the game, especially in Tarant, where you might want to consider spending a Fate point to pick their pockets for a choice firearm or other item; no spoilers here, but PM me if you want 'em.

Also, be aware that Virgil is so dedicated, that he will literally faint from trying to heal you, even if your TA is too high and the healing spells fail more often than not. Buy him a supply of bandages, or keep Fatigue restorers on hand.

There are also four followers in the game who can each get to level 4 in two tech colleges: Jayna (Herbology and Therapeutics), Magnus (Smithy and Mechanical), Vollinger (Chemistry and Gun Smithy), and Sebastian (Electrical and Explosives). This means you can have them make stuff for you, such as healing salves, charged rings, repeater rifles, etc. They do have to be in your party; you can't just go up to them and say, "Build this for me, please."

The tech manuals (available for sale at Tarant University) will be a godsend, allowing you to build from learned schematics without investing points in other colleges. They are, however, pricey and expensive; and their effectiveness is tied to your IN. Consider investing in some potions of IN to use right before you plan to build something, so that you don't need to purchase quite as many.

One last protip: You only get 64 points to play with (unless you install the Level 127 hack, which effectively removes the XP ceiling). So plan out your points beforehand; few things are more frustrating than being 3/4 of the way through the game, already at level 50, and with no more points left to finish your build.

This should get you started. If you have any more specific questions, feel free to ask.