I wouldn't put my safe/guns in the garage.

But, if your safe is a fire rated safe, a lightbulb or couple of goldenrods may provide enough heat to maintain it at a reasonable temp and keep the relative humidity from condensing on the guns. You might look into some kind of small thermostatically controlled heater to put in the safe or make such a device from hardwarestore/radioshack parts. Shouldn't be a insurmountable "mission to the moon" project.