Add an outlet to this switch?

Question

I have an exposed conduit switch box on my front porch that controls a spotlight. I would like to add a gfci outlet to the switch box (swapping the switch box for a double box thst is weatherproof) The switch seems to be a 3 way switch but the wiring does not match what I have read for either a single pole or a three way shared switch circuit. And as far as I can tell there is only one switch for this light (inconveniently located on the front porch). I do not know if the power goes to the light or switch first.

Switch has one side with a dark screw and a gold screw, then the other side has a second gold screw opposite the first, and a blank space opposite the dark screw. There is also a ground screw.
In the switch box is one run with black white and bare wires. Black goes to the dark screw "top" of the switch, white goes to gold screw on the opposite side and bottom of the switch from black. Bare is attached to the ground screw.

In one position all the black, white and empty gold screw are not hot, in the other switch position the black and empty (same side as each other) are hot but the white is not (using a noncontact voltage sniffer).
In the light box there is also one lead (pigtailed to two spotlights) Is it possible to add the gfci?

Answer 1

This is a 3-way switch being used in 1-way mode. Note that nothing is attached to the second traveler (brass) screw. That's a perfectly fine setup. With only a single cable to the switch, it is not a "power to the switch first" setup.

There is only one cable going into this light. That means it's not a "power to the light first" setup either.

Yes, that means it's something you're unfamiliar with.

There's clearly some sort of third junction. I'm getting the impression you think the switch is inconvenient for where the light is. Maybe our third junction is at a light which is appropriately located for that switch, and either you don't know about it (burned out before you moved in?) or it has been removed but the box left. Or, the box is somewhere else entirely, or in the service panel.

Regardless, all this to say, you can have the light and switch, or you can have the GFCI, but you can't have both...

And separate from all that, you can't alter the premises without permission from the landlord...

And separate from all that, you can't do electrical work on a rental unit (even if you own it) unless you are a licensed electrician.