Crossplay between Steam users and other users is by default not enabled.

To allow for Steam users to Crossplay with GOG, they will need to subscribe to the Crossplay Beta.

To do so:

1) Right click on Stellaris in the game library.

2) Click on 'Properties'

3) Navigate to the 'Betas' tab on the top.

4) There will be a dropdown menu, selecting will bring a list of options. Make sure to look for the beta that includes crossplay in the name.

5) After selecting and applying the new beta, wait for Steam to finish updating and open the game.

6) Make sure when creating a game, to tick 'Cross Platform' at the bottom when making the server name and password.

I hope this clarifies how to get Crossplay to work. This issue was discussed and addressed in a previous thread, I though it would be a good idea to make a new one to allow easier access to the solution for new players.

Previous thread:
www[]gog[]com/forum/stellaris/cross_platform_multiplayer_with_steam_friends

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last update moved the option to the launcher

All you have to do is go into “Game Settings” in the launcher and scroll down to “Cross Store Multiplayer” and click “Launch” as shown below:

If for some reason that doesn’t work, you can also add “-nakama” as a launch option in Steam. Do this by Right-Clicking Stellaris, Going to Properties, General Tab and adding “-nakama” to the Launch Options. This will force the game to switch from the Steam multiplayer backend to the new one for crossplay purposes!