I assume it will be against at least one wall. Put two bolts in the back floor of the safe and two lag bolts near the top of the safe into a wall stud. Safes are mild steel so they will be easy to drill through the back wall at a stud location. Anchoring the safe to the floor and the wall at the top will give the best mechanical advantage against prying it loose.
If a crook has enough time to get a safe anchored as above loose with a crow bar and move it out, there are easier methods of getting into any safe.
Also, a gun safe FULL OF GUNS is a lot harder to move than an empty one!
MERRY CHRISTMAS Cobb.