The Gunner is similar to the Hyperion Sniper type enemies from the main game. They carry long range weapons which come equiped with red dot sights so you are aware of their presence. Whenever you see that red line, make sure you get to cover because they are remarkably accurate with their weapons which tend to have a rather high power rating.
Once you get into cover, they are quite easy to deal with. While they are very accurate when aiming for a body out in the open, they are still remarkably poor at the fine adjustment of trying to hit an enemy in cover.