Big dumb question of the day.

[Rust_shackleford]

You'll be okay

[BPTactical]

The Golden Rod will easily counteract any vapor moisture that may make it into the safe when it is open.
Your not in California anymore, winter in CO is usually pretty dry humidity wise.
While heating the garage sounds like a solution don't forget that colder air will hold less moisture than warm air will.
Throw a couple of 1# cans of silica dessicant in the safe if it bothers you.
Really about the only time you really have a worry in CO is if you run a swamp cooler or have a damp basement.