Anything is better than them being in ambient conditions.
I would go with something like this. Highly rated, has a gauge to tell you when it is full of water and when it's full of water you plug it into a wall socket and overnight it dries it out. Many of the reviewers have put them into gun safes.
http://www.amazon.com/Stack-On-SPAD-...productDetails
Also consider getting a humidity gauge for the inside of the gun safe, or if you want to spring for it one inside and one outside if you want to know how much drier it is inside vs ambient humidity in your house.