Does it happen that games on GOG are deleted? I noticed in the community wishlist that "Return to Castle Wolfenstein" and "Carmageddon" are marked as COMPLETED. However, they are nowhere on this website. So it seems they were taken down.

Yes, from time to tome some games are removed from GOG catalogue. But when it happens they're not removed from your shelf if you bought them.
However, in your case Carmageddon and Return... are not available because you are from Germany. Sometimes regional restrictions are applied.

Thank you very much, I didn't know about regional restrictions on GOG! Also good to know that generally games are not online here forever, so we shouldn't wait too long. The German laws make me sick, unbelievable. The German version of Return to Castle Wolfenstein is heavily censored and problably Carmageddon as well. Thank you for the tip about gifting the games!

Wasn't always like this, but when they officially entered the German market with German language website and forums, they figured they should start adhering to German laws now.

As for games remaining online, it's not that they remove games on a regular basis, but it can happen due to license issue. They usually give fair warning when something like that happens so you still get a chance to buy it before it goes offline (sometimes they even do a special sale for it), but not always, and you might miss it if you don't hang out on the forums that much.

(Incidentally, the last case was just a few days back, when Alan Wake had to be removed from all online stores due to expiring music licenses.)

It's not totally ontopic and even though I doubt GOG cares, it does no harm to vote here.

Thanks! There is another, but I don't know how to make a link here. I think many Germans here don't know of these petitions, otherwise they would have voted.

GOG don't remove a game because they want to (say, because they think it doesn't sell well enough), but only because the publisher tells them to. In the issue of rights issues, it's not even that the publisher want to remove it, but because the publisher no longer has rights to sell something that is used in the game - say a piece of music, or the depiction or logotypes of real-life organisations/people - if the game is changed to no longer contain said content, it can most likely be sold again.

It doesn't have to be rights issues though, the owner may decide for another reason to no longer sell the game here (one example being Armello).

You're welcome.
Here is the other link too! :-)

EDIT: To be able to post a link, your reputation should be over a certain number, which I don't remember. A workaround is to post your link inside a quote tag.

Maybe, but I'm almost sure, the first time I've been directed to the age verification wish was through the German forum on GOG.

I just want to add that even though you can't buy some games, others can gift them to you. So if you really want a game, ask someone trustworthy and make a deal with him/her to buy you said game(s).

I just found "List of Games removed from GOG" on gogwiki.com

Yes, those are games which aren't sold anymore. The lucky people who bought them before removal however still have them in their accounts.