Someone was having a problem with this, but my internet broke in the midst of my helping them, so for them and anyone else with this problem;
When you buy NWN from GOG they give you a generic key that everyone else also gets, naturally it is often in use. If you want a key you can use online without a problem you need to request one.
The support page to do this at is: http://www.gog.com/support/contact/technical_issues_with_games/neverwinter_nights_diamond_edition You must be logged in!
(http://img94.imageshack.us/img94/8132/e9cc51319823bcff6738808.png) (//%22http://imageshack.us/photo/my-images/94/e9cc51319823bcff6738808.png/%22)
In the "Problem Type" drop down menu choose "multiplayer key request", put the same subject and a tactful message. Click Send at the bottom of the page. A key should arrive by e-mail within a few days. Ta-da.
(http://gyazo.com/e9cc51319823bcff6738808db4e0df73)
Stickied.