Retrofitting safe to protect it from fire requires a multifaceted approach because you have a few objectives:
- Prevent direct flame impingment on any portion of the exterior
- Limit temperature rise within the safe
- Protect against damage from firefighting activities
- Make the safe somewhat accessible
- Keep it cheap
The idea of covering your safe with gypsum board or Type X (fire rated) gypsum board is pretty good. Gyp board performs well because it is a fair insulator with a high moisture content; the effect of the embedded water is to require all of the water to boil off before a temperature wave can propagate to the unexposed side. Water has a very high latent heat of evaporation which means that it dramatically slows the progress of heating. If you ever saw a temperature profile versus time of the unexposed side of fire-exposed sheet of gyp board you'd see the temperature rise slowly to 100 C (212 F), stay there for a good long while, then begin to climb more rapidly. That's the effect of the water boiling away.
What makes Type X better is that it's typically thicker than regular gyp board and also has fibers placed in the mix that allow the board to retain its shape after all the water has been driven off. As any fireman can tell you, once gyp board is exposed to fire, it calcinates (turns to chalk) and loses all strength. However, that's more of a problem for fire spread through failing walls of a home.
Behind a layer or two of Type X gyp board I would recommend a thick layer of flexible fiberglass or ceramic insulation such as Fiberfrax. Fiberfrax has an extremely low thermal conductivity and is non-flammable. This will keep the temperature inside the safe lower and be especially useful in the hinged region. A dehumidifier inside the safe should be used for such a tight configuration.
Placing the safe atop a few cinder blocks is a good idea to keep it out of water.
I would prefer the basement for this sort of storage. It keeps your safe out of the way, away from thieves, and is less likely to injure someone in the event of a home collapse (fire, earthquake, tornado, nuclear armageddon, Hillary presidency, etc.).
If you were a little crazy and really intent on your guns surviving a fire, you could make the door and other holes water-tight with gaskets or sealant and have a water jet spray on the safe, triggered by a smoke or heat detector. Flammable fuel storage tanks are protected similarly against exposure fires. This may be the most effective option, but it is the least reliable --- very few people know how to wire a smoke detector to an electronic switch, few people remember to change the batteries, the water supply could be turned off or the power might be out. But it could be a fun project if you've got some extra pipe lying around.