A bunch of people, but I'm not one of them. Most of the people who used to update the various parts of the wiki got tired of it; most of them don't really even post in these forums anymore.

As for myself, I'm just continuing to update the OP of this topic because I'm still checking the store and/or forums most days (so I'm likely to see threads mentioning games being removed), and it's not much trouble. But if anyone wants a properly cited, up-to-date list like the wiki used to be, they're probably gonna have to do it themselves. (Also, the GOG wiki's continued existence going into the near future is very much in question, last I heard.)

I've always been bummed that I never got a chance to purchase Chessmaster 9000 on GOG before it was pulled. Which, admittedly, was a long time ago now (2012).

Good thread.

The Telltale fiasco has (hopefully temporarily) struck down The Walking Dead series.

Announcement here:
https://www.gog.com/forum/general/the_walking_dead_series_removal/post1

The games affected are:

The Walking Dead: Season 1
The Walking Dead: Season 2
The Walking Dead: 400 Days
The Walking Dead: Michonne
The Walking Dead: A New Frontier

(Also The Walking Dead: The Final Season, but it had already been removed from sale before this.)

Added to my list, now that they're actually removed.

The only one I missed that still kills me is Second Sight. I waited for it to go on sale and
instead it went away. :/

May be a little unrelated, but the issue that I had would sound more like a bug rather than a permanent removal. Has anyone of you noticed how some games would disappear from the GOG catalogue only to reappear sometime later? This happened to me with the Sid Meier's Civilization series, where the game Civilization 4: The Complete Edition literally just disappeared from the catalogue when I searched for it (only the first Sid Meier's Civilization appeared after the search). A few days later, or one day later perhaps, a GOG user told me that the game (Civilization 4: The Complete Edition) was never removed from GOG. Also, games such as Galactic Civilizations and Civilization 3 also disappeared from the GOG catalogue when the whole thing had taken place.

The first Civilization is not and has never been on GOG. Perhaps you're mistaking it for Sid Meier's Colonization?

Civilization III and IV were definitely not removed from the catalogue. Another option could be a bug in the search function.

Ahhhh thank you!!!! You solved my problem!!! I just realised that I had typed colonization instead of civilization. Thank you thank you.

don't worry when thq nordic bought timesplitters they also grabbed second sight, it will likely come back at some point

I just went to see if any of "The Gamers" movies were on discount during the recent sale and had trouble locating them. Always wanted to get the complete set but not for the asking regular price, and now I see they have been removed. :-( Just curious if anybody knows when specifically they were removed, and what the reason for doing so was?

They were requested to be removed by the creators due to a fairly recent incident on twitter with GOG's official account.

I just noticed that the removal of Startup Company this year has gone unremarked on either the wiki page or this thread. Other "In Dev" games have made the list, so I'm guessing it was just overlooked...

Source (from dev):
https://www.gog.com/forum/startup_company/startup_company_leaving_gog

Source (from GOG):
https://www.gog.com/forum/general/startup_company_will_soon_be_delisted_from_our_catalog/page1

As Lord_Kane alluded to, there was a Twitter PR kerfuffle, and Zombie Orpheus Entertainment (distributor of the first couple The Gamers films, and producer of the later ones) decided to take umbrage over it and pull everything of theirs from GOG. As for when, it was a little over a month ago.
I think there's a full list of titles affected a little bit upthread, and they're all mentioned in the OP, too (in their own subsection of the 2018 entry).
Overlooked, indeed. As in life, so in death, apparently. =P
Thanks for bringing this to my attention! Now I just have to figure out where in my list the game would go, chronologically...
EDIT: That was pretty easy, actually -- its removal apparently "fell between the cracks", in that period between when Grargar stopped updating the wiki page, and when I started trying to take up some of the slack with this topic.

Thanks, both of you, for explaining what happened. I did first seek out that Wiki page that lists all the games removed, but I believe it contains only games and not movies, which is why I decided to post here. If there is a version of that wiki list that shows the movies can you point that out as I had trouble finding one.

On a side note: per chance do either of you know where else those movies can be acquired? Watching those were on my to-do list, and I got the first part when it was given out for free a number of years ago, but I did not want to start until I got the entire set, and with parts 2 - 5 no longer on GOG I am now discouraged from starting since I will not get the complete story. :-(

The GOG Wiki is pretty much wholly abandoned at this point. I think the removed games list was the last page that was being kept up to date, and Grargar gave up the ghost on that back in about February. (Since then, there's also been a warning by the guy that runs the wiki that the site might go away at some point, though for now it's still up and, last time Grargar tried, editable.)
I know of no other equivalent list; this thread, unfortunately, is the best we've got for now, until and unless someone else tasks themselves with making a proper, up-to-date chronicle (and at this point, I doubt it will be me).

First off: I haven't seen any other than the first film, but it's my understanding that not all of them are related to earlier entries in the series in terms of characters or story; some (maybe most) are just kind of "thematic sequels", using the "The Gamers" name and trafficking in the same types of humor and concepts. So you shouldn't expect anything like a single, coherent meta-plot thread running through the whole series.

With regard to finding a place to purchase the films: [search engine of choice] is your friend. =) I imagine the discs are available on Amazon.
The price may not be great, but the Zombie Orpheus website would be a place to check...though I don't know about the DRM-freedom of their store's digital downloads, so if you specifically want to buy just unfettered video files to download, that might be a taller order. (One would hope that, since they sold them DRM-free through GOG, they would do so through their "official" web store, too.)
I know at least some of the earlier films are available in full on YouTube, and have been for years; I've had Dorkness Rising (the second film) languishing in my "watch later" playlist for at least a couple years now, and I first watched the original there, as well.