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.