Living in the humid Gulf Coast of Texas I would never give serious consideration to placing a gun safe in the garage.
The garage is the place meant for hot water heaters,washer and dryers,cars,motorcycles,and lawn equipment.
Not my safe or guns.
If the garage was my last and only option I would build a storage closet around the area the safe would be placed and would insulate it,run power to it,and drop a small A/C vent tapped off my main A/C trunk to isure a normalized interior humidity of my own living quarters.