Does anyone know if there is crossplay between the GoG and Steam versions? I want to buy it in GoG, but I'm scared of the potential lack of online in skirmish.

From their FAQ:
Q: Is there cross play between the Steam and GOG versions of Warhammer 40,000: Dawn of War - Definitive Edition?
A: Yes! Both platforms can play multiplayer together in any way (automatch, custom games, etc.) though players cannot use friend invites across platforms.

It it still possible? I can not see their games and they can not see mine.
We also seem to see different games in the "join games" list.

Yes GOG and Steam users can play with and against each other.
But on thing also has to apply, everyone must use the same language of the game.

If you play the german version, you would only see other players with the german version, french only french and so on. It's best to play the english version, most players use it so you can find the most games online!

THANK YOU! :D
Once we had the same language it worked. :)