Isn't Galaxy only required, if you want to play online MP (aka: "the full content")?
But if you have no interest in online-MP, you are free to simply ignore Galaxy completely?
Therefore, the use of Galaxy isn't mandatory for all customers?
In that case, calling it "required", is kind of an oversimplified statement (even when connected to the "the full content" addendum).

Edit:
GOG can really do what they want - it will always be seen as wrong by parts of the customers.
Over the course of many years, lots and lots of customers asked for the option to play online MP...so GOG enabled that feature via Galaxy.
And now other customers are calling them out for "betraying their initial business statement", by selling games that offer their players online MP.

No. Unfortunately there are single player games on GOG where either Galaxy or being online to get some content. This might be some DLC, that only works with Galaxy or some 'bonus content' (extra missions or cosmetics), that is only available if you connect to some third party account. The degree varies, but I compiled a list of cases that I know, so that everyone can decide for themselves, what degree of DRM they are comfortable with:

https://www.gog.com/forum/general/drm_on_gog_list_of_singleplayer_games_with_drm/page1

Yes, but the problem it seems to me, is folk coming here and expecting the Steam experience, but a GOG variant. They can kind of get that using Galaxy, but not really if they are also using Linux ... not out-of-the-box anyway.

And by many reports, Galaxy compares poorly with the Steam client experience, usually just on speed alone.

Ultimately GOG aren't really helping, so it isn't surprising that some like the OP get confused. We can't really blame the OP too much ... though much is revealed if you just put two and two together, and don't rely on assumptions.

Maybe GOG feel that keeping the water muddy gains them more sales, who knows.

I could blame the OP a little bit, but I would rather blame GOG instead for giving him the expectation, or perhaps not removing that expectation, of a Steam-like experience. I would just be treading on old ground here about Galaxy's inherent inadequacies that you know and could explain better than I do, so I'll leave it there.

To be fair I was not looking for a Steam like experience, I wanted to download the game without having to use a launcher. If I want to use a launcher then I just use Steam. With GOG it all went wrong with slow download speed + slow installation speed, especially with slow installation speed. That is when I started considering going back to Steam and doing so proved to be a good idea.

I shouldn't have mentioned not being able to use Galaxy, because even if it worked on Linux, I would not use it. That is not what I was looking for in GOG.

If it's GOG's strategy to promote Galaxy by having bad installers then in my case it is not working. I just won't use GOG then.

I'm not even gonna address the fact, that, in that thread, you simply lump everything under the title: "DRM on GOG".
Going by just the opening post, and a few games mentioned there, that I also own...honestly: most of these "issues" really don't bother me.
I know, I know - we're not all alike and I won't argue over personal preferences.

The "Securom-bug" in "F.E.A.R." would be something, that bothers me - but the game starts for me, so...no issue.

Some of these really seem to be nothing but "bait", to lure players, who "need" everything, to go online.
Since I'm not one of those...the baiting doesn't work on me.

Some other "issues" also seem to regard stuff, that one has to look up online, to find out, that it exists in the first place.
In "Battletech", I don't remember ever noticing, that I couldn't use certain "decorations and emblems" in offline play.
Maybe the game "rubs it under your nose" - but I honestly never consciously noticed. shrug

In regard to the CP2077 map: I can simply open that in my browser (https://maps.piggyback.com/cyberpunk-2077/maps/night-city) and access it, without having to log in to anything.
Now, when I played the game, I never made use of that map, but shouldn't [Alt]+[Tab] be the quickest resolve to this "issue"?

The one real issue I agree with, is in regard to "Hitman (2016)", in your second post.
THAT one was really DRM-infested and kept most of the SP game from you, if you didn't go online.

But again: in that case, we're talking about "most of the SP game" - not just cosmetics and/or purely online MP related stuff or stuff that is the equivalent to "you want this shiny horse armour? Then join the dark side - we have cookies, too".

And that's the reason, why that game isn't available anymore here on GOG.
I guess, it would still be here, if GOG had no problems selling DRM-ed games, don't you agree?

GOG din't have an issue with selling Hitman as it was here. What forced them to remove it was the outcry, which in that case also spread to the social media. So it was a big enough issue to be noticed publicly, which forced GOG to react. Before the outcry, GOG tried to ignore the issue.

There were even some games on GOG that were fully DRMed - and GOG never did anything about it. Luckily the devs reacted to player protests and removed the DRM through patches. (examples: Beat Hazard 2, Cult of the Lamb)

So no, GOG has no problem selling DRM-ed games. But there are still enough players who care about DRM here, to create some pressure to keep the games mostly DRM-free.

You are missing my point.

GOG cannot be like Steam, in any way except via Galaxy.

So you cannot compare your experience, GOG versus Steam, certainly if Galaxy is not in the mix.

The Offline Installers is a very different thing, compared to the direct download from Steam.
The download from Steam is a stream of files to the install location.
With the Offline Installers you are getting a package, that needs to be downloaded first and then extracted.
That extraction, depending on game size and how heavily compressed, takes time, sometimes lots of time.

GOG is or should be all about backing up your games, because that is the benefit of DRM-Free.

DRM-Free is not about a quick download and install, except via that Galaxy option, which is a stream like Steam's, and not truly DRM-Free, though usually DRM-Free Lite.

All that said, you do have a good point about the download speed for many GOG customers, which is extremely poor for many of us. That is definitely a failing of GOG, and for many a good reason not to use them.

Slow download speed is so true. GOG's download is pretty much unstable. Sometimes it's fast, some other times it's barely downloading.