So I've seen elsewhere that Neverwinter Nights 2 doesn't work on 64-bit systems but does the GOG version work with them? I think it was specifically Storm of Zehir that had problems with 64-bit systems for some reason. So can anyone confirm it all works on a 64-bit OS before I buy it?

Thanks for your help!

Yes.

I am running Windows 7 64 bit, and I have GOG NWN1 Diamond Edition and NWN2 Complete. They both run flawlessly. It is not a Windows issue in the majority of cases where people have problems with games, it is hardware and/or drivers. Of course, that's not to mention those who don't RTFM.

Running both complete with no issue on Vista 64

Windows 8 here. Working fine for me.

Other than having to run NWN 1 in Administrator Mode, I can't say I've found any difficulty with Win 7.