I think the GFCI is working properly, esp since you said you had no problems before with other equipment plugged in.
A long string of christmas lights, CHEAPLY MADE, has dozens of open air exposure to the conductors, place these exposures next to your home, add moisture from the air, you have a tripping GFCI. Wrapping lights around a tree would be especially prone to trip a GFCI.

Connect your lights to a non GFCI outlet using an extension cord. You're not going to be touching the lights while they are plugged in, anyway.

Unless you've already stapled the lights in place, you can put the string of lights in a plastic garbage bag, the GFCI should no longer trip. Indicating your lights are the problem, not the outlet.