-Nameless and Deadshot are both very good options. The key is to find fire resist uvs to compensate for the lack of natural resistance. If you are an inexperienced gunner or get owned on the mask phases you might want to lean towards nameless for the easier shielding and elemental resist. Once you are good at dodging the zombies and avoiding dmg make a deashot set to maximize your dps.
-ASI on guns only effects the gun raise and lower speed. At first this might seem weak but if you avoid reloading by raising and lowering the gun before the clip empties asi makes a huge difference. Last i remember you get around 12 additional shots a min with very high asi on a polaris. Also asi is crucial to using autoguns as it reduces the long post-firing animation that leaves you open to trojan swings.
-For FSC gunning you only need blitz and polaris. You can also bring a nova or storm driver is you are a good with thier charge shots. Alchemer damage output is very dependent on the skill of the user so use at your own risk. I generally bring Blitz Polaris Nova and Valiance when doing vana or swap out the nova/valiance when heating gear.

Hope that helps