Cashii: The game comes with the key already entered. You can look it up in the game. Go to Options -> Miscellaneous -> Enter your CD key.
We all have the same CD key. Connecting to the online server works, but none of us can play with each other unless someone changes his or her key.
GoG are apparently working to fix that. My guess is that they will supply people with other valid CD keys, but at the moment the only solution I can see is that you change the key to another working one yourself.
Where would I get another key? I assumed that buying this game would give me a key like I get when I buy a retail game. When will there be a fix? I was going to buy this, but I guess I will wait until it is usable.