Some turrets have an overheat element just like those found on vehicles. Does the gunner mod improve stationary gun overheat, or does it just work on vehicles and improve walking speed with detached turrets?
When using mounted anti-infantry guns, the weapon overheats less quickly, allowing for longer firing at the maximum rate of fire. Also it allows the player to move faster when the weapon is detached allowing for more offensive tactics rather than defensive.
I am yet to unlock any speciazation to get access to this ability to test it out but if the quote above it to be believe then yes, it does affect stationary guns (MG Turret, Shade).