Dungeon this dungeon that... I don't think there's any word more overused in game titles.

Please keep in mind I haven't played the game myself - all that info is from the web, so I have no idea how "butchered" the final product actually feels. I'm still waiting for more reviews. RPS already have one here, by the way:
https://www.rockpapershotgun.com/undungeon-review

Unfortunately, parity with Steam is far from the norm on GOG, even for new releases. Just look at the Google Docs list in the first post of this thread, in case you were unaware:
https://www.gog.com/forum/general/games_that_treat_gog_customers_as_second_class_citizens_v2/page1

Shadows: the Fallen Dungeon (called after a severe collision problem that makes the characters fall and fluctuate forever)

Not sure I understand this statement. Sure there may not be as many sales here as other places, why is this a reason to restrict what is available to customers here, do they pay less? Is there a significant cost in providing he same service to customers regardless of platform? If so then perhaps they should not be accepting the cost, or sales, and focusing on one platform? Just sounds a bit “we like the extra income, but can’t be bothered with any extra effort”.

Protagonist style and animation reminds me somewhat of Skullmonkeys. Kinda "clayish". Love it

I think that may be it. Look, I don't claim to understand why soundtracks and artbooks for new releases are regularly (!) missing from GOG. But consider the fact that the EGS also typically doesn't sell such DLC for new releases. And the EGS is doing everything it can to become a true competitor to Steam - surely, if they were allowed to sell the DLC, they would. So what explanation is there, other than "devs/publishers/artists can't be bothered to put this extra content on stores that can't promise the same number of sales"?

You forgot ".... Chronicles"!

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EDIT: I just realised something interesting: The Kickstarter project had originally planned for the game to release on PC, Mac and Linux on Steam and GOG. The Kickstarter had a stretch goal for an XBox One version but they didn't reach that goal.
Now, the game has been released on Steam, GOG, the EGS, XBox and GamePass, but only for Windows. So they cut builds they had promised from the beginning (Mac/Linux) and content stretch goals they DID reach (character-specific quests) while using development resources on stretch goals they DIDN'T reach (create an XBox build)...

Pretty sure it's not an actual requirement. It's more of a guarantee up to what point the developer/publisher vouches for you to be able to play the game. Actual requirements might be much lower.

It's like a "best before:" on a pack of salt.

Is the demo mentioned in the trailer only available on Steam?

Clicking the demo on Steam takes me to the full game. It may have only been available prior to release.

Like "Dungeons & Dragons", "Dungeon Master", "Dungeon Keeper" etc.?

Indeed, "shadows" seem to win this one. "Fallen" I think takes third place. "Souls" is an ambitious newcomer doing well, but still has ways to go.

Dungeons Souls (that's twofer!), Dungeon Siege, Dungeon Rats...

Right now it seems that the most generic thing one could make would be a pixelated roguelike called Fallen Souls: Dungeon of Shadows.

Of the three mentioned by you only Dungeon Siege has reached a certain age.

Out of boredom I looked in a Thesaurus for alternatives and found this: torture chamber, cell, oubliette, vault
The alternatives don't sound that much better.

Not sure what you mean. I didn't say anything about age?

Well, the ones I mentioned were older then a decade. Like Dungeon Siege. Nothing important, just an observation.