I agree, a gun safe is not as safe as a real safe. (phew say that 5x fast!)
Gun safes have a lot of hype surrounding their marketing, but if you cut to the chase and look at the UL listing, at best they're rated as an RSC or "residential security container"-- and though they're heavy and they have thick walls that's usually insulation with a thin metal surround. The insulation can be as flimsy as gypsum (ie drywall) and in some cases can harbor moisture. Do the research, gun safes are not as good as a real, steel safe.
Also if you have any guns, ammunition or cleaning supplies in the gun safe I for one would be worried that the various chemical compounds involved shooting or cleaning the guns would be harmful to the coins. I have no actual information on that, but there's so many variables there that it would make me nervous.