From the 1.2.55 changelog:

"« Other news »
Recently, the Family Fireside Fables were also released. It’s a pack of six charming tales read by the game’s
unmistakable narrator, Ed Kelly. You can listen to it for free on the platform of your choice (see YouTube & Steam)."

I own the game on GOG. The platform of my choice is GOG. Why is the DLC available on Steam, but not on GOG?

Isn't it automaticaly installed in the game?

But I think you misunderstand the update.
For THIS additional content the available platforms are Steam and youtube if I understand the text correctly.
Steam chose to waste storage-space on a non-game content, while the others still can get in through YOUTUBE, which is obviously not a gameing platform.

So for THIS non-game content you can NOT choose the platform as GoG, you must check it on youtube, as stated.

That's exactly my point. You can call it "non-game content" all you want, but since the publisher chose to release it as DLC for the game, it would be appropriate to consider it game-related content. Regardless of what you choose to call it, it is unethical and unacceptable for the publisher to make this content available on Steam, but not make it available on GOG. Making it available on YouTube is not an acceptable alternative, as YouTube videos are not stored on your computer and cannot be watched (or listened to) offline (which is one of GOG's selling points -- being able to download your game and game-related content and subsequently access it when you're not connected to the Internet).

So, you get that your question is pointless, and you DID get the DLC in the described way of the DLC, you just choose to be a troll.

If you want, download the bloody youtube videos, it's no big deal.

No, the DLC is NOT available on GOG, therefore I did NOT get it (in any way). Do you even know what DLC is or are YOU choosing to be a troll?

Asking for DLC parity between GOG and Steam is not "pointless", nor is it being a troll.

Let's use an analogy. Let's say you purchase an album on iTunes, and the exact same album (same name, pictures, UPC/ID code, etc) is available on Amazon for the same price. Several months or years after your purchase, the label decides to update the album with additional tracks, but provides the update only to Amazon, not to iTunes, so you don't get the extra tracks. The label then says "Hey, you can buy the album (again) on Amazon, or you can listen to the songs on YouTube" (where you must be connected to the Internet, have a decent connection speed, agree to Google's terms of use, and accept the fact that the extra tracks can be removed at any time). Many people (I would guess most people) would not be happy with that situation.

As for downloading the YouTube videos, that would be illegal, and believe it or not, some of us still respect copyrights.

Your analogy is bogus at best.
It is more like selling hardware in IKEA, but providing the manual not in print, but on a webpage that hosts manuals.

Get this: it's not even a GoG vs Steam issue. Children of Morta is on a lot os platforms and devices, like Nintendo Switch, éplaystation, whatever. Anyone but Steam gets this suprlus material which does not effect the gameplay at all on youtube.
Steam offers not a regular, gameplay-effecting service with this, but a premium service.

This is not some platform war again by Steam where non-steam customers get jack shit, or get the stuff half a year later.

PS: youtube itself even offers downloading tools, and it's very much in legal borders, you are just not aware of fair use and stuff.

Since GOG is willing to "waste storage-space" for such non game content as soundtracks, wallpapers, art books, "making of" videos and whatnot I see absolutely no reason why shouldn't this be added as bonus in extras.

No, my analogy was completely relevant and appropriate. Your analogy, however, was completely bogus. In order for your analogy to be similar, the same hardware would have to be offered at the same price at another retailer, and the hardware must come with a print manual when sold by that other retailer.

I never said this was a GOG vs Steam issue. I said asking for DLC parity between the two storefronts is not "pointless" or being a troll. That holds true, regardless of where else the DLC is or is not made available.

As for your point about copyrights, yes, I do know about fair use. Duplicating copyrighted content for personal use is not an example of fair use. In fact, it is one of the things copyrights were created to prevent.

if you wanna compain related to that, ask instead why you must buy games to listen to the soundtrack? They have nothing to do with the games functionaly.