Well, since ranged damage is based upon DEX, you'll want plenty of it! DEX gears will help, if you're rich, go for Gloves[1] with Sting Cards (+1+4=+5 DEX each) and a Stem Worm Card (+5 DEX, garment card). If you're not that rich, an Apple of Archer will do as well
Like a Hunter, you can go AGI, for faster attacks (especially Handguns, I think, since there's something like Double Attack).
But Gunslingers seem to have heavier attacks, so ASPD would not be first priority. Then go VIT for more HP and defence.
LUK helps for the Flip Coin skill, but I never saw a Crit Gunslinger @_@
Oh well, I don't have a Gunner, tested one once a year ago on the Beta server
But hope, it helped ya a bit anyway
