All games are Steam-only.
Steam(ing pile of crap) Bundle strikes again.
The DRM-free tag on Humble means there is a standalone installer. No Steam required. If both tags are present, there are both standalone installers and steam keys included. I feel safe buying DRM-free on Humble. I don't use Steam.
Well...sometimes you get an actual standalone installer. Other times you just get a zip file containing the game content. I really wish Humble had some standards for their DRM-free downloads, but they just seem to treat them as an afterthought.
I was pretty sure I've seen DRM Free yet only available on steam, maybe one of those idiotic things where people claim games on steam aren't DRMed - or I may remember it wrong.
In any case, I've had the same issue as above. Sometimes there isn't really an installer, and I've had games that just don't work right. Suffice to say, my experience with GOG is in general much, much better, but unfortunately they only support relatively few Linux games.