I tried searching through the forum and the GOGmixes but I wasn't successful (or maybe I did it wrong):
I'm looking for a list of GOG games that have Christmas mods and/or Christmas levels.

Or, in case such a list doesn't exist, which GOG games do you know that have these things?

I know that Giana Sisters: Twisted Dreams has Christmas levels.

Edit:
I put stuff into a GOGmix:
http://www.gog.com/mix/games_with_xmas_levelsmods

Hmmm... I know for sure that I had the Halloween event with the GOG version as well. :-)
Thank you!

From the "What did just update" thread:

NOTE! CHANGED Escapists, The, changelog (http://www.an-ovel.com/downloads/chg/chg_1423221839.htm)
NOTE! ADD +++ Patch 1.2396 (10 December 2015)
NOTE! ADD +++ Adds the Santa's Sweatshop Free DLC

Sounds like Christmas stuff to me :-)

Duke Nukem 3D has an entire campaign suited for these needs.

Sacred 2 has a Christmas quest-line. However, you need to use the Community Patch in order to still play it, because the devs removed it again (or rather, replaced it with a different seasonal quest-line: i.e., Easter-egg hunt).

Tyrian 2000 has a Christmas mode during December.

Telltale Game of Thrones

Winter is coming!

H2H-Xmas should work with Doom II. Nuclear Winter is so awful. Unless your needs are of the sadomasochistic variety it's worth skipping entirely.

Thank you, everyone! Please keep'em coming!

I so need to have a look at Christmas Tyrian 2000, I like the game a lot!

I'm really bad at The Escapists and Duke Nukem though. :-p

Witcher 1! Merry Witchmas module!

Does Santa still show up in X-Wing?

Space Pirates and Zombies (+Bounty Hunters) has special Xmas mode.

Edit: Nevermind. You already mentioned it in the OP.

Doom 2 has been mentioned. Let's add Quake. http://quakeone.com/files/4-mod-files/147-twisted-christmas/

I recall there being more than that one.