For that I needed the key (obviously) and the Ubisoft login data.
I went ingame into the VENICE Addon (not the original) and created an online profile, which I could succesfully link to my ubisoft account with the key I got from GOG. I could copy the Multiplayer Key from Gog (go into your library, on the game, then under 'more -> CD Key' you find it) I bought the Anno1404 Gold Edition via GOG Since I had quite a discussion with the Ubisoft and the GOG-Support on this topic lately and now have new issues, I wanted to share this with you: