Steam is (of course), and sometimes U-Play and Origin but I remember very few times that games from Humble Bundle were redeemable on GOG, even though they offer drm free games from time to time.

Because CDP only owns the 4 Witcher games, which aren't quite enough to create a bundle, in stark contrast to EA and Ubisoft who own lots and lots of games.

Nobody knows, it seems they just don't like giving out keys to third-party sellers.

Maybe they're afraid that while Valve, Ubisoft and EA lure in new customers and bind them to their client that way, the same wouldn't work for the DRM-free games on GOG and it would just lead to people buying stuff from Humble instead of directly from GOG, or shady traders exploiting the system by cheaply buying GOG keys in order to sell them for profit?

Valve don't own a lot of games either, you know. I don't think this is the issue.

It's not the store that supplies the keys. It's the publishers and they:

1) Prefer dealing in Steam keys
2) Not usually give more than one methods of activation.

Edit: Actually scratch part of my previous post. Since GOG is now a publisher as well, the number of games they could distribute is 18 instead of 4.

From what I remember to have read, it doesn't seem as easy as publishers just generating keys though. They have to ask GOG to provide them, and I think I remember some devs saying that they would love to give out keys but GOG either doesn't like to supply them or in any case it seems more complicated than generating and distributing Steam keys.

Why does GoG still not offer the Linux version of Leisure Suit Larry Reloaded? It's a Unity game and the Linux port is out there for years! -.-

Only GoG knows.

Maybe (some devs have claimed as such and G-Doc had said that GOG will actually give keys, provided they aren't sold alongside Steam keys), maybe not (Judas has mentioned devs/pubs can just ask for keys and they'll be handed over).

In case of Steam, you are correct ( publishers can generate own keys). In case of gOg, this is much more problematic (not easy to generate keys outside gOg). Which I guess answers OPs question also. If gOg allowed publishers to generate own keys on the same scale as those with Steam, I think more gOg games would have been in bundles.

Even if given the choice, I doubt that, at least, big publishers would offer GOG keys alongside Steam ones. Look at Nordic/Square-Enix's offerings on the Humble Store, for instance; despite DRM-free versions being available here and in other stores, they only offer a Steam key and nothing else.

Which I blame all the bundle abuse on, and is exactly what I forecasted a couple of years ago. For the those missing those options - you have made your bed, now lie in it. You showed yourself not to be trusted with such a system, so it is not strange that no trust is given to you either (talking generally here now, not to you specifically)

The only game I have seen given a GOG-key in a Humble Bundle is Defender's Quest

Humble doesn't see Steam as a direct competitor.

There is limited draw for the other bundlers to work with gog though while I'm sure they would love to have a few gog keys in there as well once in a while, there are downsides in addition to upsides for gog having keys with them.

I've gotten ones from groupees and indiegala, I think?

they were nice. they were the only reasons I got those bundles I think.

Groupees had several GoG keys. I recall some for games from the "Dracula" and "Atlantis" series.