Well from a viability perspective rogue and guns dont mesh well together as its very hard to get sneak attacks at distance as you can't flank and all.
So Slayer would offer more bang for your buck as studded combat and the bonus feats helps toward the feat starved gunslinger.
What type of play style are you going for? We need more information.
A WIS based spell caster works well if you don't mind losing a little BAB, which isn't a big lose for gunslingers. Also being able to use a cure light wounds wand will be nice in case the primary healer gets knocked out and you need to wake em up or an emergency situation.
Cleric gives several useful abilities via domains (*cough*travel or darkness*cough*)and cast using WIS from a good spell list. And as long as you didn't dump CHA too much you get at least a couple channels to perk up your party after the fight or damage haunts. Even a 5 level you get 2-3 3rd level spells. (Including at level 3, the 2nd level spell darkness which can be helpful for your later shadow dancer stuff.) In my opinion the best choice if you want a spell slinger.
Druid gives one of a few domains, an animal companion (for a 5 level dip, an animal companion is a waste), or herbalism if you want to make free potions. A few situational bonuses that you may or may not use. Druid spells aren't bad and being able to summon allied creatures can be useful. But loosing metal armor probably is a deal breaker.
Ranger gives a few bonus feat that can be useful from the archery or crossbow styles. A couple favored enemy bonuses that you can share with allies. A little magic, but 2-3 1st level spells isn't much. Good for a bounty hunter like flavor but fighter is clearly better for the extra feats.