Some newer ATI/AMD cards will try to force settings onto NWN, based off a built-in driver profile. If you rename the game's executable, and launch the game from the executable instead of the default launcher, you should be able to get around this issue.
To do this, go to the game's installation folder - by default it's C:\GOG Games\Neverwinter Nights Diamond Edition\ - and find the file nwnmain.exe.
Rename nwnmain.exe to anything else, like nwnmain2.exe and, from now on, launch the game by double-clicking this renamed executable.
You can also create a shortcut to this executable, by right-clicking it and selecting Create shortcut. You can then move the shortcut to your Desktop, or wherever you like, for your convenience.