I heard that a lot of people manage to do crossplay quite well. However, so far, it doesn't work for me (GOG version) and my friend (Steam version). We tried to do Direct Connect and it didn't work.

So, my friend who has the Steam version hosted the game. He generated a code so that I could join the session via Direct Connect. It failed: it said something like unable to connect. The same with the other way around when I hosted and he tried to join.

We then tried Hamachi for a virtual LAN connection. It worked and I could join the game via LAN connection instead of Direct Connect. However, whenever I joined the game which my friend hosted I keep experiencing de-sync issue. It became much worse to the point of unplayable in the swamp area beyond Fort Joy.

Note that we've disabled firewall and no mods were installed.

Works like a champ for me. I have the GoG version and my friend has the Steam version and, while at first I would give him the code, that got annoying, because it changes every time. So he tried my (external) IP address and it works just fine with that too. Unfortunately, FiOS likes to change that with too much frequency, but it has worked for the past month anyway.

To get my external IP, i use:

https://whatismyip.com