TheEnigmaticT: This is not part of a war on Christmas, any more than it's part of a war on Yalda, Modraniht, Saturnalia, Pancha Ganapati, Yule, or Kwanzaa. The end of an old year and the beginning of a new one is about much more than just one faith or belief, and if I'm not referencing your specific one, perhaps you can just fill in the blanks with what makes you happy.
I must say that I, as a Christian in America, was mildly offended as soon as I saw the name of the sale. I think it comes from this battle that we're having in the States (and probably elsewhere too) where employees are told they're not allowed to say "Christmas," children aren't allowed to sing spiritually relevant songs (I admit that I wouldn't want my kids forced to sing a praise song to Saturn), and lawsuits (often frivolous, but not always so) force cities to take down displays.
But when I read t he GOG description of the sale, I completely agreed. And I realized some important points:
1) GOG is an International website with a wide audience of tons and tons of faiths.
2) GOG is from Poland, a country who knows religious persecution better than any country on the planet. Surely they're not trying to persecute.
3) Christmas is about remembering the birth of Christ and giving one another a present to remember both the present of Christ from God and the presents given to Christ at his birth. Most of us are using the sale so that we have games to play for the next few months (though all of my purchases have been for the treasure hunt RPG I'm building:
http://www.gog.com/forum/general/request_for_donations_of_gog_codes_for_treasure_hunt_rpg/page1 -- PS if GOG staff wants to throw a few games into it, let me know [Witcher 3 for the grand prize? wink wink?]).
So how is buying a game for ourselves have any relationship to Christ?
What this sale is about is end-of-year sales, meeting budgets and proving fiscal fidelity to the powers that be. Not to celebrate and honor the miraculous birth of Jesus.
4) GOG is great with free speech. Just look at these forums. Very little moderation (save the removal of spam) is done here. And I'm sure they'd have no problems with anyone, even their staff, saying "Merry Christmas."
So, all in all, I don't see a problem here. But I probably would have without the description of the sale. It was wise to put it there, and perhaps I learned a little more about being a world citizen because of it.