Found on gog forum:
You will need to change your cd-key if you are playing multiplayer either on LAN or the internet.
Your multiplayer CD-Keys are stored in your owned games list, click on Neverwinter Nights 2, and they are listed under "My serial number:"
To manually change your CD-Key, go to where your Neverwinter Nights 2 was installed and open 'nwncdkey.ini' it will looking something like this (Obviously I have changed the keys to 0's):
[CDKEY]
Key1=00000-00000-00000-00000-00000-00000-00000 //This is your base Neverwinter Nights 2 cd-key
Key2=00000-00000-00000-00000-00000-00000-00000 //Mask of the Betrayer key
Key3=00000-00000-00000-00000-00000-00000-00000 //Storm of Zehir key
Replace the keys here with the corresponding keys in your GOG owned games list.
En français (ayant la version d'origine, je n'ai pas téléchargé le pack de GOG, mais apparemment il y aurait un utilitaire GOG dans lequel, sous la rubrique "owned games", se trouvent les codes en question) :
- ouvrir le fichier nwncdkey.ini (qui se trouve dans le répertoire d'installation de NWN2 - attention, pas celui de "Mes Documents", mais bien celui de c:\program files etc...)
- remplacer les codes existant par ceux trouvés dans la liste GOG.