After all the digging around I did trying to optimize the Gundolon (and Algunist), I figured I had a good handle on things.
Still probably missed something though.
Summoners have no Prestige options as well.
The best option there is to homebrew something along the lines of "if you would gain '+1 level of arcane casting' you can instead have '+1 level for purposes of Eidolon stats/abilities/Evo. points'".
Going strictly by (current) RAW, Gunslingers are boned.
Going by 'reasonable interpretation', most things that work with a CROSSbow should also work with a Firearm.
I specify crossbow, and not bows, because... well, lets do this:
Firearm: Simple point and 'click' interface, easy to fire one-handed.
Crossbow: Simple point and 'click' interface, easy to fire one-handed.
Bow: Both arms must work together to aim and shoot, difficult, if not impossible, to use one-handed.
I can see a re-fluffed Arcane Archer working with Firearms, but as-is it is VERY set on the flavor of bow/arrow.