The thing I don't understand, is that Steam games that use Galaxy for cross-multiplayer, like Space Hulk Deathwing, don't need Galaxy installed. Using the same logic, shouldn't the GOG versions of those games also not require Galaxy ?
Okay that doesn't make any sense in my head. What are you even trying to say? "Steam games using Galaxy for crossplay don't need Galaxy."
games DO use Galaxy for crossplay, but also DON'T use Galaxy for crossplay at the same time? Which is it?
Are you saying Steam-Games can crossplay with GoG-Games, without having GoG Galaxy installed?
If so, then yeah, obviously. That's the point of crossplay.
The opposite of that would be GoG-Games can crossplay with Steam-Games, without having Steam installed.
And yes, that works as well. Which is the point of calling it cross
So... simply put.
Devs make a game. That Game doesn't do multiplayer if it's not programmed to have multiplayer. But programming multiplayer takes time and money.
So now Valve offers a plugin puzzle piece that comes with Steam and can just be slotted into place by the devs, instead of them having to invent their own code. So the Devs never do their own multiplayer code anymore, but just put in a puzzle piece provided to them by Valve. That makes it 1) easier and cheaper for the Devs to have a working multiplayer with little effort, leading to more games with multiplayer and less work for the game Devs, and 2) it also makes Steam nearly mandatory on the market for gamers if you want to play anything with multiplayer because without Steam, games simply can't do multiplayer anymore.
Back in the day, "dedicated servers" were much more common to come with the game, and the game did everything "multiplayer" on its own. But all of that stuff has been removed and replaced by that puzzle piece now, and games heavily depend on it.
So when you then remove anything that used to be "Steam" from the game, the game is now also missing that multiplayer ability puzzle piece. Therefore, GoG either has to expect every Dev to develop their own multiplayer code for a GoG release, or accept that GoG games simply won't have any multiplayer. That, or GoG themselves can provide a puzzle piece to slot into that already existing hole, and make multiplayer work exactly like it does on Steam, so that GoG-piece now also has to work with a client just like Valve's does, which GoG then calls Galaxy.
Thus, multiplayer capability has been restored for a game that had the Steam puzzle piece pulled out of it, as without that piece, the game can't do any multiplayer at all. And only then, depending on which agreements have been made, that multiplayer also may or may not support crossplay with Steam as well, but that's a different topic entirely.
So ultimately, GoG Galaxy even existing is Valve's Steam clients fault. Steam took dedicated features out of the games, and integrated them into itself. Both parties liked that for the reasons stated above, and it streamlined the multiplayer experience to the players as well. But it also had the result of games becoming unable to offer multiplayer without Steam. There are still a few out there that stick to the old ways of dedicated servers and utilizing their own multiplayer code, rather than using Steam's plugin. Many of these games have their own accounts and / or launchers, you can browse for Servers to play on inside of Minecraft, Factorio and Project Zomboid, and even host your very own dedicated server for others. But most games only send invites to play with a Steam-Friend through Steam, and use the Steam-Plugin to connect and run "their" multiplayer. That needs to be replaced somehow, and that's what Galaxy does. Friendlist, invite to multiplayer, code to connect and run it. It's a replacement plugin for the hole Steam's plugin left in the game.