I disagree.

I may be wrong, but it seems like he’s confused about how a gfci works and is only checking for moisture at the outlet itself. He also hasn’t mentioned trying any other load besides the one set of lights.

So OP, if you’re reading here, unless more than one set of Christmas lights causes this trip, you probably have moisture getting into your lights somewhere. Could be the female side of the lights, could be a loose or broken bulb.

If a few different loads all trip it, then definitely change the outlet.

Take out the drywall of the whole span from a stud that isn’t in your way at the beginning of your run to a stud after your run. Reframe the area with a “beam”. Use the first stud that isn’t in your way as a king, put a jack stud next to it, cut material out of the two studs you want to go “through” to the height of your jack stud plus the width of the beam (5 1/2, 7 1/4, feel free to overdo it), place another jack next to the far king stud. Double or triple (depending on wall thickness) up a 2x6 or 2x8 for your beam that will rest on your jack studs.

Basically like you were putting in a window or door. Just a normal header.