They make gun magnets, super-powerful rare earth magnets that will hold several pounds, encased in plastic with screw holes so you can mount them to the bottom of something like a desk or cabinet. The gun attaches/detaches instantaneously and discretely, no combination required, but even an earthquake wouldn’t dislodge it. Put it someplace she would never have reason to look but you could access it walking past casually without attracting too much attention.
I know a guy who installs secret safes for a living. He only ever deals with one person and when he does the install he makes sure the only other person in the house is whoever paid him. And that person is warned never to tell anybody where the secret safe is. That’s a great role model.
While you’re at it, solve your problem. Take the spare tire out of your car and leave it someplace where she’ll have to walk over it the next time you two go somewhere. When she asks about it tell her you aren’t planning on having a flat on that trip. You know the road and don’t expect any debris to be on it that could cause a flat.
She’ll naturally call BS on you and when she does you ask her what’s the worst could happen if you have a flat and don’t have a spare. The answer is, you’ll have to call AAA to come rescue you. Or if your phone has no bars, you might have to walk a bit to get some reception to make the call.
Then ask her what’s the worst could happen if you NEED a gun but didn’t bring it.
You can get over needing a spare and not having it. NEEDING a gun and not having it, ... maybe not.
Great approach! Bravo