I read that multiplayer is being developed.
Will be MP cross play? For example, steam players will be able to play with gog players?

I also would like this, to play with other Steam friends, avoiding to purchase the game twice

As far as I am aware, GOG version runs only in a "non-steam" server mode, which means that it does not use Steamworks to connect people with each other. But the thing is, that you can setup the steam version to use non-steam server too, I have done it before.
So do not worry, you actually can use both versions together, you just need to set up the server differently (there are plenty of guides how to do so)

Thank you, I will look for a guide

If it's any help, as I tried to do it few days ago - to make sure it works right, run the steam version of the game with a "-nosteam" launch option (it's under properties of the game). This way it will surely see the GOG version of the game, as it runs only in nonsteam servers version. Hopefully it will work, good luck!

Seriously, crossplay is a "must": most people will rather use steam version because of higher online, and even there people say online is low.

No, errors that you have not a steam version.
Note that you can run a steam server without the steam client and it Downloads the workshop mods without a steam login.
Thereof it seems like DRM preventing you from downloading the Mods. Which the dev's could fix to handle steam mods downloads too, if they cared.
And there so many good mods that are steam workshop only. So its kinda a kick in the face.

Hello,

has anyone successfully tried the cross-play with a steam version yet? A couple of minutes ago we were not able to find each others servers and we've called it a night for today.

My friend owns the game on steam and has started his copy with the "-nosteam" option to avoid using steam servers for multiplayer. I've set my server up as "public" and the game showed me my public IP address and Zomboid default port 16261. But still no luck with the connection.

I have even preemptively tried unblocking the Zomboid default port in my router (UDP), but we still can't find each other.

Does anyone know what I might have missed?

As far as I know, Steam players can play on non-steam servers with the launch option -nosteam. Although, GOG players cannot play on steam servers though, because it requires steam authentication for security, friends, workshop, etc...

So the answer is, it depends on the server. If it is a non-steam server: YES if it is a steam server: NO

We've actually managed to do it. But we needed a little 3rd party help in the end. But I'll come to that.

First I followed this comprehensive guide:
https://steamcommunity.com/sharedfiles/filedetails/?id=514493377

This looked very good and I launched a server on my computer. But my friend was still unable to connect. Further analysis indicated that the default ports for Zomboid seemed to be still unreachable for my friend (we used https://www.yougetsignal.com/tools/open-ports/). That was odd because all port forwarding settings on my router looked okay.

Then I stumbled over an article that suggested, that my internet provider had to change a setting for my connection to allow port forwarding. But the nice lady at the hotline told me that they are not blocking any ports.

Frustrated, I mused over the problem for a few days. Then I remembered a tool I used years ago to play a different game online: Hamachi. It creates a VPN peer-to-peer connection between computers over the internet so for the computers it looks like they are in the same network. Using Hamachi is dead simple and there are some nicely written guides available online so I won't get into details here. [edit] Just one thing: When you want join a server you have to type in the Hamachi-Ip-address displayed in the Hamachi window for the hosting computer into your Zomboid server-join dialogue. If you have any questions just ask here. [/edit]

In conclusion that software was the key to our success in the end.