Sometimes, you need to make a 360 Graus view of the air battlefield in order to find Planes from where the coming.
BUT. the gun move slower that the head of your soldier, the suggestion is simple. Make when you press B (’'tank commander weapon stance") to move his head without move the Anti Air Gun.
plus. if possible, make he do so whit his Gun, because now AAA have 0 firepower against inf, anyway if the gunner become able to look around whitout move the AAA gun, it’s a lot progress by the way.