I think giving gunners a passive buff is the way to go, but I wouldn't give them anything that buffs damage, directly or indirecly, because that would either be too insignificant or too strong and ruin the game balance.
There is one thing that would make the gunners have a real advantage over engineers:
-give gunners more firing arc with all weapons, maybe a 20-25% more?
This would be something really really useful, that potentially allows for a much greater offensive capability but still depends on the gunner skill to be used effectively.
What do you think?