Gun "safes" aren't really safes, rather they are big steel lockers that theoreticly provide protection from theives.
If you want fire protection, you need to buy a safe that is designed to protect the contents from fire.
A safe provides fire protection in 2 ways, first by delaying the temperature from reaching the contents via insulation, and secondly, by sealing the contents off from oxygen. Safes are rated, on the door for 2 time periods, the first being the number of minutes the safe will require for a semi skilled burglar to get into it using a specific set of tools, and the second, being how many hours the safe can protect it's contents against a 1700 degree fire.
In today's world, there is very limited market for used large safes, so, if your floor can hold one, and you can get the safe into your building without paying a moving fee, you can generally get a 6 foot high double door for hauling it off.
One word of caution here, safes do NOT provide humidity protection for guns, so a lot of guys think it's a good idea to drill a hole into the safe and install a heater inside. This is NOT a good idea since it provides an oxygen path and voids the fire protection value.
If you do buy a new insulated safe, make sure it is thoroughly dryed out before putting your guns in there. Most "safes" are shipped from the factory with the concrete insulation still wet, and it will cause guns to rust.