To play Neverwinter Nights online, you will require a unique CD key. You can find it in your Account on the website:
Once logged it, select the game from library, click on MORE -> SERIAL KEYS.
Please input it in the Multiplayer Key Changer that you can find in the Start menu, in GOG GALAXY, after selecting Neverwinter Nights, under MORE -> Other -> CDKey Changer.
With the changed CD-key, premium modules (like Kingmaker) will stop working as they do use the default key for validation. To get around this, go to your My Account page, select the game from the library, and find the premium modules re-installer, under the Game Goodies content. Download this file and install it. The modules will now be adapted to the new CD keys.
Please note that Neverwinter Nights 1 and 2 master servers are no longer functional. You should use the Direct Connect option to log onto your preferred server via IP address.
The current list of active servers can be found here: http://www.nwnlist.com/