Apparently controlling the humidity in a gun safe is quite important.  I don't know this from personal experience, but at countless trade shows over the years gun owners passing by my booth have looked at the desiccants on display and told me they can be used to keep a safe perfectly dry.  

Here's how it's done. 

1. Fill a muslin bag with blue (or yellow) indicating silica gel beads.  About 1.5 to 2 lbs of desiccant beads will fill a 5 x 7 inch bag, which Amazon sells in 25 packs for 11 dollars.
2. Place 1 or 2 sacks in the safe.  
3. Periodically open the muslin bag and inspect the beads.  If the beads have turned pink (or green) they're saturated.  Throw out that bag and toss in a new one.
4. If you're looking to save money, regenerate the saturated beads by putting them in the oven for about 15 minutes at 350F.
5. Store the excess beads in an air tight container.

I offer a 25 lbs pail of blue indicating silica gel beads for $135. That price includes freight (to US addresses).  The great thing is that this pail has a sealing lid.  So you can safety store your muslin bags of desiccant beads for later use.