Original NWN for Mac was compiled for the PowePC architecture, so it would run using a PowerPC emulator on Intel Macs. At some point this was removed from modern Macs, so NWN for Mac would no longer work on Macs, so it isn't sold anywhere anymore.

For NWN 2, it looks like Aspyr, who developed the Mac port, lost it license for it, so the Mac ports for NWN 2 were taken down from everywhere.

So this is why original NWN and NWN 2 for Mac are not on GoG (or anywhere else).

Now Beamdog has got the rights to update NWN 1, and have recompiled it for Windows/Mac/Linux and it is currently available only on Steam.

Your options are:

Buy NWN:EE on Steam.
Wait for NWN:EE on GoG.

Get Windows versions and play through Wine or dual booting to Windows.

With a little setting up, the Windows version of NWN2 and its expansions from GOG runs well on a Mac using Wineskin. It requires the directx9 and vcrun2005 winetricks, and for me at least tweaking a couple of display settings to run full-screen by default (otherwise the mouse pointer disappears after scrolling). Once that is done it runs better for me than the Mac port used to. I play on a 2013 Macbook.

If you're not familiar with Wineskin you can read more at http://wineskin.urgesoftware.com. This is a fairly easy game to get up and running so may not be a bad way to learn about Wineskin if you're inclined.

Best wishes.


EDIT: NWN1 for Windows runs well on Mac via Wineskin, too. It doesn't require even the tweaks needed by NWN2.