Pg. 46- "Shooting with Artillery"
Gun models cannot be fired if they moved at all in that turn's Movement phase- they cannot make Snap Shots.
Otherwise, one crewman that is within 2" of a gun in the shooting phase can fire it.
So, because the crewman is the one firing the weapon they may still fire at flyers and flying monstrous creatures, this rule just means that the gun itself is not allowed to make Snap Shots when moving, yes?