Wil lbe my first mod so I don't know how to do this sort of thing yet, i found out how to get them to drive and stuff though! Pretty fun this modding stuff.
You have to make a seperate team for each gunner. Then you assign that person the "Cover" properties. Then you place the actor real close to the gun (or the cover "X" right by the gun), and he should hopefully use that instead of his personal weapon.
Where the stationary gun is positioned, place a defence station with the highest priority. In the plan for your actor, lock his stance to upright, set MROE to all costs, CROE to suppress, and then cover. Set the cover arc to be the same as the defence station arc (or as close as you can get it). Then just place the actor right behind the gun.
Could you have a plan where there's a "PATH" leading the bad guy right up to the gun, and then include those other things (Defend Station, Cover) at the end of the path, on top of the gun...
Would that make him run to the gun and then use it?