All variants of multiplayer/coop this game offers can only be played via GOG Galaxy!

I'm really p'ed that I fell into that lousy "read the damn fine print!" trap!!!
If you wonder: You can find the tiny note below the "System Requirements". Have a magnifying glass ready; I failed at that when I bought the game in november.

Thank you! You saved me from purchasing this game and having to refund it.

BTW, the note about requiring Galaxy is only there on the Standard Edition's page. The Slayer Edition's page doesn't include the note at all, so it really needs to be added.