Hey, its been a while since I played this game....
Ok to the point: I have a suggestion for the gunner but I don't think this idea is relevant enough to put into the "Gameplay" section so I put it here.
In a lore sense, I would think that if you are called "a Gunner" you would know the make-up, composition, and mechanics of any gun there is out there better than anyone.
So in that case, a Gunner would be more knowledgeable about a Gun than an Engine and an Engineer more knowledgeable about an Engine than a Gun.
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I suggest a Gunner be able to Repair and Rebuild "Gun" components faster than engineers.
Not sure how to integrate that with Gunners being able to use Engineering Tools, if Stamina would take that into account or how fair it would be for Gunners to have a passive ability dedicated to them.
Just thought it was a neat idea.