The Gun Tank's bad and the base gunslinger is excellent at doing damage. Not as good as the musket master or pistolero but still very good.
With 20 point buy I might choose Str 9, Dex 16+2=18, Con 14, Int 10, Wis 16, Cha 7.
If you're not going to multiclass then for feats consider
1: Point blank shot
Human: Precise shot
3: Rapid Reload
Gunslinger 4: Rapid Shot
5: Deadly Aim
7: Clustered Shots
Gunslinger 8: Weapon Focus (your main weapon)
9: {whatever}
11: Signature deed (something)
There are other variants like going for two-weapon fighting with pistols. I could detail this but be aware it's very specialised.
It's been said a number of times that gunslinger doesn't offer enough past level 5 to justify continuing with it. Gunslinger 5 / Fighter 6 or G 5 / Inquisitor 6, or something complicated like G 1 / Savage technologist barbarian 5 / Weapon master fighter 4 / Battle shaman 1, would all be better in many ways (including damage with guns) than a pure gunslinger.
+1.