urknighterrant: The serial keys are not needed for solo play, but they are hard coded into the MP.
I just tested it out, I deleted the "nwncdkey.ini" file (a file which contains your CD keys in plain text) and ran the game. It asked for my CD key. I clicked cancel and it exited the game.
NWN had the CD check DRM (securom based) removed via official patch years before gog (because securom kept on making the game crash); heck it was before the Diamond Edition first came out on DVD... However it still retained cdkey dependency for online play, single player, and premium modules.
Online play: The master server has been taken down. As such the only way to play is for servers to be hacked (in a way shown in bioware forums) so as to use the CDKey as a user identification token. When you log in it shows you the characters associated with your key, there is no username and password and AFAIK it doesn't have to be a valid key, it just has to be a unique one. - Justified to keep. But it would be nice if the game was modified for direct connect online play to be smoother and use a login and password handled by the servers rather then CDkey (would require modifying the server code... note that the servers come with the diamond edition and all existing world servers are hosted by players not by companies)
Solo play: It still has the DRM for checking CDKeys fully intact and I don't see any justification for it. It is not a deal breaker (I still bought it after all)... but it is disappointing and I expected more from GOG.
Premium module play: Despite promising all the extras, only half the premium modules come with the GOG version... only those which had their DRM subdued by the kingmaker. The DRM is still there, it merely emulates a DRM server locally which makes authentication very rapid. However it is tied to the CDKey used to install them. As such, modifying the cdkey (such as changing it from the generic one that it installed with to the one given to you by GOG) prevents the premium modules from authenticating.
Yes, the DRM on the online play and the DRM on the premium modules is incompatible and you can only have one of them work at a time.
GOG did not remove a single IOTA of DRM from this game, they simply repackaged the diamond edition as is with all of its DRM methods intact. And the only bit that could actually be justified in keeping was the online play