Switch – What to Do When There Is No Hot Wire in Switch Box

It sounds as if the cable going to the switch just requires a feed.

I am not sure how you intend to bring power to the gazebo from the main house, but basically you would wire both 12/2 cables to the #12 feed wire and wire it color for color; white to white, black to black, ground to ground.

The GFI would get wired to the LINE terminals only.

Please comment if you can add any details that might help.

I'm just thinking out loud here. It is hard to state anything with certainty in a situation like this where there is no guarantee of what somebody else did with the wiring a long time ago...so forgive me if this suggestion isn't even worth what you paid for it. ;)

From what you have said here, it sounds to me like that red wire makes its way back to the lights, and acts as the "hot" for the lights. Who knows where the lights are getting their neutral side connection. Doesn't really matter. Obviously it is getting there somehow.

So to make the lights light, your existing switch is getting "hot" from one of the two black wires, and is supplying that to the red wire.

So think of your red wire as the "hot out", and ONE OF the two black wires is your "hot in".

So you need to use a meter to see which one of the two black wires is your "hot in", and treat it that way and cap off the other black. Then use your red wire as the "hot out".

Again, this is just me thinking out loud about what I would try. I take no responsibility for any bad stuff that might result from it. All possible disclaimers apply. ;)