A gun safe should not be located close to the door, or in a place where it is visible to others as you take the firearms in and out. A corner of the garage where any potential thief has limited access is ideal.
Power tools should not be kept in the same garage as the safe, as this just helps any thief break into the safe. Cans of paint, paint thinner, gasoline or any other volatiles should not be kept near the safe, and neither should any lumber or wooden objects.
A fire is likely to do greater damage to the firearms if it burns hotter near the safe.
Rust is a destroyer of firearms, and a unheated garage may allow moisture inside, especially if the safe is against an outside wall. Use a wand-type dehumidifier or a gel desiccant within the safe to keep moisture away.