I guess I can't feel that strongly about which way the multiplayer support is being designed for any given game (except when it is crowdfunded by promising in advance certain forms of multiplayer...) because I can't future-proof my friends and without them I am ironically far more likely to enjoy being a loner among many strangers in a huge persistent world than playing with a few of them on any smaller scale of multiplayer.

But enough about that as this thread is more about the Galaxy client being needed even with single player, so here is my take on it:

We can "thank" Steam for creating the environment where most developers seem to expect GOG to provide them similar APIs to even consider releasing their games here and while that is unfortunate, it is what it is and the best we can do is to get GOG to have a proper Galaxy-less testing procedure where the offline installers are tested on Galaxy-less computers that are restored to that state from an image between each test to be sure that the single player side of games launches without any Galaxy related libraries or registry keys that could otherwise have been left from previous tests as it is far more likely that GOG can get developers to fix such oversights if they detect it before release than if it takes weeks or months before they get a ticket from a customer who has run into that issue.

If it's a multiplayer game, DRM is just the issue we have to accept.

Very well put! Unfortunately I do not have much optimism that "non-Galaxy" versions will ever be a priority. Honestly, it often feels like we are very lucky the non-Galaxy offline installers are even still existing. Another, perhaps even larger effect of Scheme's virtual monopoly is that it conditioned so many people to "need" a client for gaming. It is not a big leap to assume that's why GOG focuses so much resources on making Galaxy 2.0 "the mother of all clients" rather than on their unique selling point of offline installers. It is why we see Galaxy receive "special treatment".
May I ask why? DRM-free multiplayer without any client or even any third party account is possible in several forms. Why settle for less? Especially when this is the "DRM-free" store.

Depends on the type of multi-player, newer (and especially popular) multi-player games require a connection to a server which will verify and authenticate before allowing access to the online game. Many of the more modern games also have anti-cheat layers which install onto your system (which again can be seen as DRM). These layers can also cause issue and stop you from playing such as those you try to play on a Linux platform but the anti-cheat falsely flags the Linux kernel as a cheating program.

Older MP games that can do direct connect don't need a client or server connection but it does require more work on the player side to get it running.

Some people though don't see the server login as a form of DRM though as it acts as convenience so they don't have to do the work on there end to connect to an online game while others do see it as DRM and would rather do the work themselves.

In other words, it's like I said. DRM-free multiplayer without such things is possible, as proven by the older MP games you mention (and many newer indie games, and some but few strategy games). I am not trying to argue that modern games don't have these things, only that they are unnecessary.

Some people don't see Scheme CEG, EA O-rent-gin, or Ubisoft U-Rent as DRM either. And they find the largest DRMed platform (Scheme) the most convenient of all. So what? Their interpretations are ignoring the reality of the situation. GOG's "sister site", FCKDRM.com concisely makes the point about game preservation being affected by DRM. This is true about server login requirements, (ironically) including Galaxy login requirements.

The thing is though is that modern gaming as a whole is going the way of always online, there are gamers right now that have no idea how to install a game without a client or without needing to log into an account of some form or even care about having an offline installer because they have always done everything online and may not even realize what DRM actually is as they prefer convenience over anything else.

I know, but I personally don't see why those of us "in the know" (for lack of better term) should settle for that. I understand why the people who don't know any better accept it; but why should we? Particularly on a DRM-free storefront. I don't think we disagree, just maybe talking past one another.

Although I'm one of those DRM-free "grognards" I have to agree. If a game has certain features which only work on an online mode (like multiplayer option), might as well just accept the online client in order to run it.
"In for a penny, in for a pound", as the saying goes.

Of course, this does not apply if the game has a real single-player mode. In that case I reject that one needs the online client in order to run the game. Hence the reason why every single-player game on GOG ("Gwent" obviously would be out of the equation) MUST have updated offline installers available.

So all a dev has to do to make requiring a client and DRM acceptable is to put in some online-only gimmick? This, instead of demanding that any game with any sort of single player having true, offline single player, and even multiplayer components functioning in "independent" modes (LAN, custom server, hot seat), without requiring any 3rd party software or official server connection?
For that matter, no reason other than the desire to sell card packs for Gwent to not have independent MP either. Host just selects what cards are available and then players play.

No.
https://www.gog.com/forum/general/lists_games_that_need_galaxy_for_multiplayer_and_the_ones_that_dont/page1

UPDATE:

GOG has updated today the offline installers for "For The King", so I rebought it and retested it. I am glad to report that it now works without galaxy installed.

Congrats!

And a (slightly off topic) fun fact from me if I am allowed. Today I found the first game that won't run WITH Galaxy (no, I don't intend to make a thread about this as well ;)). It's the actual version of "True Fear Part 1" when installed using the offline installer. Galaxy won't launch it but it launches fine when launched directly. The Galaxy downloaded version DOES launch so maybe GOG finally decided to remove Galaxy components completely from offline installers?

Wouldn't it be a better use of GOG's resources to have them test properly that the games can be launched with and without the client while no matter how we install our games, as long as the version number matches, the contents of the installation folder would be 100% identical between all GOG customers until we launch the game for the first time?

We are not split only into 100% Galaxy users and 100% offline users, some use the client to update newer releases and download offline installers only for older games that are not likely to receive any further updates, but as there still is a chance that new offline installers for them may be released, being able to import the offline installed games to the client allows keeping an eye on that possibility.

By the way, did you try to verify the installation after importing the game to see if that could have fixed the problem?

Thanks for the update.

Seconded!