Most of their Mac games use DOSBox or WINE, so it probably wasn't too much effort for them to get Linux support working for most of them. Even before they announced Mac support, I ran quite a few of their games with WINE and DOSBox on OS X (their older games use DOSBox on Windows too), but it's a lot less hassle to get their configs (although they tend to be quite pessimistic about visual quality, and you can improve some of the older adventure games a lot by changing the scaling mode to hq3x in the DOSBox config that they ship).
I'm very happy with GOG - there are typically 5-10 games on my shelf that I haven't got around to playing yet. I got The Witcher 1 and 2 as a bundle and enjoyed them both, although I enjoyed the first one a lot more. They're DRM-free and let you redownload games, often with significant updates (e.g. I bought Dungeon Keeper, and they later added the expansion pack. FTL is now FTL: Advanced Edition).