There is an option when you are ingame (I think you need to hit [Esc]) and are host. Didn't check it, but my friend told me yesterday when we played together.
Of course you need to enable multiplayer for your game and have gog galaxy running in the background (this is called "optional").
The other option is to join an existing game - just check the available games and join. You can do this e.g. from your running game. Again you find the option in the menu which appears when you hit [Esc].

Using Hamachi for creating a game in LAN did not work for me at all. There are simply no games found at all.

Btw, kicking a player from an existing game doesn't seem to work. There also seems to be no possibility to make a game private (like locked with a code that you can tell your friends or something), so anyone can join. So if you want to play together with some friends right now, but no others (maybe because you are using Mumble/Teamspeak/...) you are screwed.
And the fact that kicking does not work is a huge problem if there is some kind of troll, who's not joining for dialogues in a mission. The whole group needs to be at the respective POI for a dialogue/..., so a single player can "crash" the whole party and there is no way to get rid of. I already run into such a situation.

GOG/DEV: Fix this please!

Thanks a lot for all the info. I've tried looking for games and quick-joining but had no luck at all, no games show. Maybe the Multiplayer community is not really big in Dying Light (at least the Gog-Galaxy one, I bet Steam has more players)

Tunngle worked for me and my friends to connect

how can i connect dying light to gog galaxy client?

Install Galaxy
Click on the GoG circle
Scan and import folders
Select the GoG install folder for Dying Light.

Dying Light should now be viewable in Galaxy.

Run from galaxy.

I bought an additional copy of the game through GOG hoping I could connect to my Steam version, so my girlfriend and I could play together via LAN, but it doesn't seem to work regardless of which machine is hosting. That is, the game connects, but it immediately disconnects. Running Dying Light (GOG) without Galaxy disables multiplayer entirely (as the topic suggests), making it a moot point. I've played the game for too many hours on Steam to request a refund, and to my knowledge GOG doesn't do refunds.

If anyone finds a workaround to get GOG and Steam versions of Dying Light to stay connected, please let us know.

EDIT: Note that both games were set to run in LAN mode. I've read that LAN mode for the GOG version is supposed to be enabled without Galaxy, but it wasn't for me. There was no way for me to even attempt a connection without playing via Galaxy. Again, any suggestions are welcome.

Problem solved, in a manner of speaking.

I decided to try connecting my GOG copy of the game with the same copy of the game on my other machine — by literally copying the game files over and running it. Not recommended, I know, but I was desparate and needed to know if I could connect one version with another. So I started the host via Galaxy and the client without Galaxy.

In the non-Galaxy client, matchmaking was disabled, as before . . . but then I noticed at the top-right, beneath the mini-map (where it shows your current objective) that I had no objectives. So I ran around for about 20 seconds or so, and the "co-op is enabled" message popped up again. Apparently I simply had to trigger it one more time. I immediately pressed Escape and saw that matchmaking was enabled. I found my host and connected, and it seems to be working! So, that was my fault, and I apologize for the confusion. Two GOG copies connect just fine in offline mode.

I'm not even going to bother trying with my Steam copy anymore because I also discovered the saves are compatible if you copy your save files over, so I didn't lose any progress at all. Now I just need to buy another GOG copy and all's well.

But I really would prefer a Steam-GOG solution, if anyone finds it. I don't really need or want a third copy of the game.

Me and the Wife used the same GoG copy of Dying Light for a Year (GOg's EULA is a little vague about this), Just bought a second copy on Sale now to support Techland and so Wife can access the recent events.

I have to say this is one of the best Co-Op games I've every played. You and your girl should have hours of fun with it.

As for licenses... you have 2 Dying Light licenses, so there is no real reason you should be allowed to do what you did.

Installed for "free", from where?

Their profile says 0 games. Same as the other person recently who posted in the general forums asking for help with their "free" copy of the Blackwell Bundle.

So u installed pirated gog version, since there is no DRM it will ask for connection to the galaxy but since the game is not registered on ur acc it will ask for CD KEY or something, Just buy the game on sale and support the devs.

Of course the client only supports games that you actually own, that are registered to your account.

lol, i looked at your profile by accident and it said you had 5xx games and thought ? then i read your name

yes he's a cheeky beggar asking for help from a place it has been pirated from

if i was him i would buy it while it's on sale lest he be chased up about it in court !

Yep just recently past the 500 mark, you're not too far behind