Has GOG actually fixed this?

I can only play with steady 15-20 fps.

I hoped that GOG had an idea :)

Not sure why you'd expect GoG to "fix" an issue that is Nvidia's to address.

Because GOG makes old games work with new systems?

Can't suggest anything specific either, except the usuals of remove shiny water, compatibility mode,
run as administrator, fumble around with graphics settings, this and that.

But if it's steady 15-20 fps, you can count yourself lucky. :)

...and you tried all of this?

http://www.gog.com/en/forum/neverwinter_nights_diamond/crashes_after_play_button_is_selected_on_the_boot_screen/post7

Which would still be a driver issue, not a game issue.

But; arguing on the internet and all that.... :)

Yeah, turn Shiny Water off. I think that option's been pretty much broken forever.