Where did you search that indicated the GOG version has Mac support?
It would appear that the game's page does not list Mac as a supported OS, but there are some gogmixes that state that it runs fine under mac.
There are a few problems with getting it to work under mac, the largest being that GoG can't give out the Mac version, as it was published by a company separate from Atari, and I'm guessing that Atari was the one to make the deal with GoG, so you probably wont find a mac installer here.
There are however, ways around this.
I would recommend you read up on this thread, as it gives a good overview of NWN on mac.
In particular read section 3.9, which will give you your options, which I will be willing to help you with, though I can't promise much as I don't own a Mac.
Your options are, essentially, get a VM(virtual machine, like parallels) and a copy of windows to run NWN in, or to use something like boot camp and run Windows as well as Mac, though this would require you to reboot into windows each time you wanted to play the game. You could also try using WINE, which will allow you to run Windows applications within MacOS, without owning windows, this is probably your best bet. The final option is to use the tech demo of the NWN engine with the content from the installer from GOG, this could very well not work, and would only be useful if Wine does not work(and you don't own a copy of windows).
I hope this helps :) If you have any more questions just ask.
Just to say to any Mac users surfing on here or for anyone interested.
The GoG.com version of NWN Diamond Edition works great under Parallels 6. At least for me. I can not only run all the series plus prem-mods I can use the toolset with no crashing so far on the toolset. I haven't tried to create anything yet just ran it to see how it works.
I will post my findings on trying to make something with the tooset under parallels 6 running XP SP2 English OS on a MBP 2.2 10.6.6 2GB memory with 128meg video 8600 card.
Also a free alternative is PlayOnMac (based on wine).
The game works without any big problems on my 24' iMac.