If you're doing it in the garage, +1 on disguising it as a refrigerator.
Make sure you have reasonable dehumidification in there too.
Either bolt it OR get a heavy enough safe that it cant be moved. Something in the 2,000lb+ or more range. Also, don't tell people you have a big ass gun safe.
This should do
A dehumidifier is a good idea or just open it up while your home and leave it up for a couple hours...with garage door closed.
I've read about some safes being pulled through interior door jams. Where there's a will, there's a way. It's a matter of making it unknown and difficult.
I just keep mine out on the bed....