Pretty much their whole catalogue of old games is 25-40% off. Includes things like the fallouts, divine/beyond divinity, various might and magics, etc. And this is off already relatively low prices. It's all been brought up before, honestly, but here's your excuse to actually go spend money.
All 3 are on gog, but I remember reading a long while back that one of them was far better than the other two, and it was one of the first two. I just can't remember which it is. I probably should just buy all 3 and be done with it.
The first, along with manuals and possible reference cards/soundtracks to download.
I go out of my way to buy everything I can on GoG instead of somewhere else. They really give you the whole package. Fallout even provided fan patches and high resolution patches. That makes you really feel they care about this old games and/or do their research.