The key is dew point, honest.
Goldenrod heater is cheap insurance, so that temp inside safe always stays above dew point. If it does, no condensation occurs.
Condensation can occur when you bring a cold gun into a warmer room, or if you open the safe door and expose cold guns to a warmer environment. The warmer air can contain more moisture (relative humidity can be higher at higher temperatures.) It doesn't mean it has more moisture, just that the potential is there.
When I lived in a cabin with wood heat, there was no problem bringing a cold gun inside. There wasn't any moisture in that air! Dry as a bone. However, a typical indoor atmosphere may have relative humidity of around 60% or so, and that can cause condensation on cold objects, until they warm to above dew point.
However, for gun safes mounted in a heated room, the building heat probably keeps everything above dew point without additional heat, such as a Goldenrod would provide. Then the only concern is to wipe any condensation off a gun before storing it...