If I need the client to play the multiplayer of this game, than how is it not basically a steam version? How is that DRM-free?

As already mentioned in the multiplayer topic, there is a LAN mode that you can use without Galaxy:
https://www.gog.com/forum/dying_light/multiplayer/post6
However, it is not yet confirmed if LAN mode between GOG and Steam users works.

If you see restricted multiplayer as DRM, you should know that this seems to be very common for GOG, and there are worse examples on how DRM in singleplayer and multiplayer is affecting games on GOG. E.g. Age of Wonders 3, where one had to create an online account for SAVING an offline player profile (this has been changed in an update), and although there is a multiplayer via hot-seat (IMHO obsolete), there was no LAN mode at all, only online multiplayer with need of creating an account at the developer.
Maybe you want to read carefully about a game in the future, before buying it on GOG. They changed their former attitude during the last years significantly (regional pricing, "DRM")

I wouldn't be so sure about that confirmed for LAN stuff. I opened a support ticket with GOG and this is the reply

"Hello

Unfortunately no, multiplayer is available only between GOG Galaxy users. There is also no LAN multiplayer.
Sorry for the inconvenience.

Regards
Genoan
GOG.com Support"

You can play multiplayer over LAN with just one copy, I just tried it from the same GoG installer on two machines and they found each other and played fine.

I could not get the Steam and GoG versions to stay connected so either it wont work or requires some extra missing around to get it to work.

This is not true. The GOG version has the same LAN mode as the Steam Version. It's working without any issues. Galaxy is only needed for online matchmaking.

Have you guys thought on adding a clear, visible, note on the store page about the online capabilities of the titles? There's some of us who find an issue having multiplayer "restricted" to the Galaxy client, and some of us who find a non issue being able to play on LAN games. I think it would be a great way to clarify this specific topic.

Yep. dying light has lan mode, but even the lan mode doesn't work with the steam version,
it found the players host in the matchmaking but unable to join

I bought the game on GoG and tried multiplayer. Didn't work.

I played through the prolouge to enable multiplayer, disabled the firewall on both systems and can't find any games. Neither LAN nor public.

That may not be the fault of the game.

Between:
- GOG having a somewhat smaller market share than Steam (and being full of "wishlisted!" as well. :P )
- People just deciding to turn multiplayer off (perhaps to avoid invasions?).
- The possibility of there being some invisible ping filtering in the game.

there's a really good chance that at any moment you won't be able to find a game. I've put my game up as a host and had people join, so I know it *can* work. I also know that I've never come anywhere near filling up a game so there's a definite population issue to overcome.

It is completely the fault of the game as I mentioned LAN between 2 PCs connected by ethernet also does not work.
I had to use the "fix" used for illegal copies namely tunngle to get those to a multilayer LAN game.
Also when using a VPN like cyberghost I suddenly found public games.
Something is incredibly wonky with dying lights network detection. I have no problems with other LAN or online games and as an It professional I definitely can rule out any network problems. Perhaps it is not able to cope with multiple network connections as I have VMware installed and thus many virtual adapters. But again: other games have no problems at all.

I can understand your frustration. While LAN works without issue for me between GoG versions, I struggled trying to keep GoG and Steam versions connected and finally gave up. Maybe the networking is wonky but it did work by default for me on two win 10 machines.

I want to offer to co-operate for the passing game, and getting some achievements.

Hi everyone! I'm from Argentina and would like to try co op sometime, If my/our ping allows it.
Do you think we could make a sticky topic or something to get everyone interested together in one place?
I've never used gog galaxy for multiplayer, can anyone explain how to invite another player for example