You would be making a very large bomb it if a fire exceeded the fire rating of the safe.
There are no gun safes that are UL listed for fire ratings. You have no idea if the mfg is lying about theirs.
My gun safe is supposed to be rated at 1800 degrees for 2 hrs. I will never believe it.
Keep you powder in a cabinet that the doors will blow out easily so there can't be compression of the gasses if you are unfortunate enough to have a house fire and it ignites.
I keep some loaded ammo in my gun safe but no bulk containers of powder.
My bulk powder is on open shelves in my loading room. If a fire, it can burn but not blow.