I actually purchased my copy of the Witcher 3 through Steam (before I got onto GoG) and you don't need the Steam client to run it so I don't see why you'd need the Galaxy client to run it either.

I call bullshit on the OP, this is quite clearly a user error.

This doesn't make sense unless you downloaded the old installers that included Galaxy with an option to opt-out. These were removed months(?) ago. The offline installers do not include Galaxy anymore. And if you click the big blue "Download And Install Now" button, that doesn't download installers. It basically downloads Galaxy which then launches Galaxy and forces the game to download via Galaxy. Below that is the offline installers. This also doesn't make sense. Games are not tied to Galaxy. If Galaxy was removed, and the game was left installed it should launch just fine. All GOG is doing is pointing the game shortcut to launch Galaxy. You can bypass this by creating a new shortcut directly to the exe, which for the Witcher 3 I believe is in the bin folder in the game folder.

But regardless, it still should have launched when Galaxy was uninstalled.

I have all my games installed by back-up offline installers:). I don't have Galaxy and I don't need it, even I can't install it as I have Vista..;P

Multiplayer games used to be DRM-free too.
For how long can I trust singleplayer games to *not* follow the same way?

Using a client system to aid in the facilitation of a multiplayer network connection isn't DRM. Unless you like figuring out ports, servers, IP addresses, and netcode.

Well, so what is the difference between having to run Galaxy to play Gwent, and always-on DRM?

There is a big difference between not including DRM in single player and using client based API's to facilitate MP. You have to go out of your way to include DRM in single player, where as client based MP API's is to make adding MP easier on the developer. LAN and Direct IP, etc fell out of popularity long ago, and is the reason it isn't included in most games today and client based MP makes online gaming more secure and helps to prevent cheating, etc.

Markets change, times change. Let's not conflate two separate issues.

Well, why don't you turn on airplane mode and find out ;)?

(Hint: One of these will let you still play.)

Because always-online DRM is more applicable to games that were primary offline games that used a online connection primary to check ownership. Meaning even single player requires a connection.

Gwent is more akin to an MMO, it's was designed primary as an online game from the beginning and uses an internet connection to facilitate online play rather than check ownership. That is the difference when talking about always online DRM.

Actually, it's both - gamers at large weren't computer savy anymore because operating computers became easier and thus they were too dumb to operate any lan or direct ip connection and were glad that they only have to click TEH BUTTON plus devs were glad that it was like that so they could cut the customer empowerment out for good and give them more power over usage of the game.

the only computer savy people where where a few people, and internet was still in it's baby stage during those years in 1980s 1990s so not everyone had that luxury
Quake 1996
Age of Empires 1997
unreal 1998

from me because of when i was born it's harder for me to say because i never directed connected to anyone same with lan since i was just born in the 1990s. people where also more infavor of a server instead of direct connecting to someone. computers in general have become a lot easier since windows 95

MP went client-based for convenience. I don't like it either, because anything mandatory third-party is the death of game preservation. Services die, APIs change... Like I wrote elsewhere, people will be able to play Quake deathmatch in a hundred years. Newer games? Questionable... But this part of the deal is up to the actual devs - they need to implement the network stuff and not rely on third party services. It's laziness on their part, if they don't, and laziness seems to be en vogue.

SP however doesn't need internet connections, it's just you and your machine. There is simply no excuse to dongle an SP game to a service, other than greed. It requires active involvement to implement DRM to restrict the rights of the customers.

Every single player game runs DRM-free on the dev's machine. The client implementation is actual additional work.
On the other hand - client implementation might be a short-cut to save work (=money) for MP.

So there's your difference. Do devs invest work to empower the customer? Or to keep them on a leash.

I wouldn't call it laziness, if your customer base is shouting from the rooftops "what is this lan shit, I WANT STEAMWORKS" you just.. do it, and then don't do the lan, because the 3% of oldschool marketing share (gog should actually cater too but are too busy being steams little brother) of people who won't buy because of no lan be damned.
Kinda like how no sane software business is targetting linux desktops.

Maybe... but offering "connect $IP_ADDRESS" as a console command and a dedicated server players can connect to is always an option.

I mean, the code is already there. There is existing server code and there is low-level connection mechanics. It's just hidden from view.