At least is is for me. I just spent the evening trying to get co-op to work with my off-line installation of Mechwarrior 5. All I was getting were unable to connect errors.

Then, before giving up on it all I decided to download and install Galaxy.

Galaxy didn't detect my already installed copy of MW5 but when I launched MW5 manually from its shortcut, MW5 must have detected that Galaxy was running and signed in because on the title screen I had a offline mode indicator flash in the corner of the screen for a second before being replaced with my GOG username and a code.

Co-op functioned perfectly after then.

Launching MW5 without Galaxy running causes it to revert back to previous behaviour.

tl;dr

Mechwarrior 5 on GOG requires you to be signed into GOG Galaxy for co-op to function...

at least for my installation. Your mileage may vary, but but don't count on it.

Further experimentation from other users for confirmation would be helpful.

Starting my copy while Galaxy is running, whether through galaxy or just the executable, gives me an invite code popup. Closing Galaxy and starting it from the executable, gives no invite code popup.

As for having Galaxy detect manually installed games - it doesn't do so automatically. You need to manually point it to the relevant folder (look for scan and import folders), which it will verify, then add to the list.

wow this sounds like bullshit, the kind you have to purposely plan out and implement

There is a fix for it now!
https://www.gog.com/forum/mechwarrior_5_mercenaries/windows_7_black_screen_workaround_for_latest_build_version_11315_51575

I... don't think that's related...? Bit of a late response, but...
Yes. Because it has been. On purpose. By Steam.

Steam popularized the "oh, let US provide a multiplayer module for you" model.

1) Devs didn't have to develop their own multiplayer anymore. (easier and cheaper)
2) Devs didn't have to host their own servers anymore. (cheaper)
3) Devs could invest the time this freed up into other things in the game. (Polish. ... well, theoretically.)
4) Steam got "multiplayer exclusivity" tied to their platform just by providing the copy-paste module and servers.

This changed the landscape of multiplayer gaming because it was a no-brainer for the Devs.
Since then, that approach for multiplayer integration bound all games to the specific store service.

And GoG fell victim to that change when it happened, for quite a long time. "Why would I get that game on GoG if it costs the same and doesn't even have multiplayer?", people asked, exactly as Steam planned for it to happen.

We didn't get ANY multiplayer thanks to Steam's success in changing the landscape in that way, until GoG managed to write their own replacement multiplayer module that, by design, was forced to work in very much the same way, including needing to be provided by the Galaxy client.

GoG could only try and adapt, and I'm glad they actually managed to do so. That way, Steam's multiplayer module could just be swapped out for GoG's multiplayer module with minimal effort by the Devs for a fully featured release on GoG.

There are a few games that still do have their own multiplayer, and it's either a nightmare to set up manually (Zomboid) or requires you to create an account with the Devs/Publisher (Factorio). That's the options we're left with, apparently.

So: Jeez, thanks, Steam.

But to be fair, yes, it also turned out to make sense for most other parties involved. Even...
5) More convenient for players to get into multiplayer with their global friends list integration. Click join / invite. Done.
6) Indie Devs can use it to more easily create games with multiplayer than ever before, without it.

So, uh... actually wow, thanks, Steam. I guess.
So I got mixed feelings about it.