Mmh, I think it works like this:
1. To have internet-multiplayer you have to have Steam installed and running (be logged in).
2. Then run Dragon Commander (maybe add to Steam as a non-Steam Game -> left lower corner "Add Game" -> "Add a non-Steam Game").
3. The Game should now detect Steam running and use Steam/Steamworks, the internet multiplayer tab should be available.
Keep in mind, that is my understanding how it works, I don't use the GoG.com-Version so that might also be wrong
