Also:
"A few things to note:
1) The game should come with a single-player CD Key.
2) To play Neverwinter Nights online, you will require a different unique CD key than what comes with the dowload. Please contact gog.com through
http://www.gog.com/en/support/contact and select "multiplayer key request" in the "I have a problem with:" drop-down menu. They will send you the CD key along with additional instructions. Changing the CD key from the single-player key to the new online key will result in the premium modules (like Kingmaker) not working anymore, because they will use the default key for validation. To get around this, go to your My Account page on GoG.com and find the "premium modules re-installer", under the game's additional content. Download this file and install it. The premium modules will now be adapted to the new CD keys." - BTB
Laz.