Id check the firearm securtiy regs in your area, but my guess is they only say something in the order of "attached to the building" so you should be fine having it elevated...& if thats the case, id do it for sure--better than everything rusting away...!
For any moisture barrier, id use DPC/Forticon--its the same black stippled/embossed plastic used in brick walls.
Dont use waterproofing membrane (paint on) unless you know what you're doing as most membranes are only effective for positive or negative pressure (meaning they only work one way & so may decouple/peel off what you paint them on).
Or just have it elevated...just keep in mind that it will be easier to pry the safe off the wall if not against the floor, so it might be worthwhile putting extra fixings in the wall.
If you only need it raised 40mm or so, personally id just sit the safe on top of a concrete paver with a layer (or 2) of DPC between the paver & safe, & not drill through to the floor at all--as it will only encourage moisture to travel up into your safe eventually.
Id run the dpc up the back of the safe 100mm or so, or run a bead of nuetral cure silicone along the abutment of the paver/wall & pres the DPC into it while still wet--this will stop moisture rising up the wall behind the safe.
If you do this, you will want to make sure the fixings in the wall allow for no upward movement of the safe (do this anyway)--ie, the screws or bolts are hard against the steel of the safe to the bottom of the hole.
I dont know what construction your wall is (timber, steel or masonry), but there are ways of ensuring the safe has no upward movement with either screws into timber wall of dynabolts into masonry wall--basically try to angle them down slightly ensuring they are hard against the steel of the safe.
Best of luck, let us know how you go...
The man who knows everything, doesnt really know everything...he's just stopped learning...