Metal Garage Ventilation

Air humidity is related to temperature. If you heat air, the relative humidity drops, all things equal. But amount of water vapor present is the same.
Adding vent air won't really cause trouble in the summer unless you've got stuff in the barn which somehow gets below the dew point, which would cause water vapor to condense onto those cooler items.
Venting between the roof and the insulation helps you cool off the envelope and maintain a lower relative humidity.
It doesn't sound like your location/climate comes with a significant risk by just adding Gable vents. Considering most metal buildings are about as air tight as our cars (not at all), trying to vent between the roof and insulation is probably not worth the cost to setup such a configuration if your building currently has fiberglass sandwiched between the roof and framing.