Why isn't the Witcher 3 in the list of GOG connect games?
I bought a key for the Witcher 3 season pass on Humble Bundle which I thought would give me a steam key. I already have the base game Witcher 3 on Steam. I didn't see the little GOG icon, my mistake I know. When I connected my steam account, I was suprised to see that the Witcher didn't show up. GOG is made by cdpr right? Why wouldn't they have connectivity support for their own games?

You misunderstand how GOG connect works.

GOG connect doesn't give you Steam codes for GOG games. It adds games to your GOG account that you already own on Steam.

And even that only works for select games for a limited time because the publishers probably wouldn't allow this otherwise. So it's basically just a form of promotion.

As far as I know the only way to get The Witcher 3 on Steam is to buy it directly on Steam. Everybody else (Humble, Amazon, etc.) is selling GOG keys.

The only thing you can do is try to get a refund from Humble and buy it again on Steam.

While The Witcher and The Witcher 2 can be registered on GOG if you have it on other platforms, there is not yet such a system for The Witcher 3. There's also no indication of if or when it may be enabled. Perhaps we read the post differently. Notagreatname has a GOG code (from Humble) for The Witcher 3 Season Pass, and The Witcher 3 on Steam - I understood it as he would like to get The Witcher 3 on GOG so that he can use the Season Pass code, but since it doesn't show up in connect (or has any other way of registering on GOG), the best potion would be to refund the Season Pass and get it on Steam instead.

Interesting... it seems that Humble Store doesn't stock any Witcher 3 Steam codes at all. I didn't expect that. So I guess you can't ask Humble's support staff to swap your GOG key for a Steam one.

GOG probably won't be able to give you a Steam key, but you might have luck contacting CD-Projekt and asking them for help. Who knows - maybe they might be able to give you a Steam key for the expansion pass or add the base Witcher 3 to your GOG account so that you can use the code here. No promises though - if they're avoiding selling Steam keys on Humble they might not be inclined to hand them out, but it can't hurt to ask.

Or you could ask Humble for a refund, if you haven't already.

It's the same with Ubisoft's games. Although many of Ubisoft's games are available on Steam, if you buy a key elsewhere, you will inevitably get a UPlay key instead of a Steam key. Ditto with EA/Origin, Microsoft/Windows Store and probably eventually Activision/Battle.net. Microsoft have been publishing some games in a delayed fashion on Steam (Halo Wars), some see rereleases by the developers themselves (Rise of the Tomb Raider, Quantum Break) but some will inevitably remain Windows Store only (Forza Horizon 3, Gears of War).

Welcome to the beginning of the post-Steam era - where distributors want ever increasing slices of the pie and so seek to control every element of the supply chain. Give it five years (or less) and all Steam will be good for is shovelware (not that it's really much different now). You cannot be a serious PC gamer by restricting yourself to Steam.

When you release a game on Steam, you can generate Steam keys and sell them wherever you want (see paradox plaza, for example).

Both CDPR and Ubisoft doesn't do that, because they already have their distribution service, so you can only buy from the Steam store if you want the game on Steam.

So it's not the Humble Store, it's CDPR that doesn't provide them Steam keys (but they do provide GOG keys, for obvious reasons). Actually, I'm glad to have my library scattered in multiple services. I prefer DRM-free, of course, but I'm okay in having more clients installed, it reflects the freedom of pc gaming. I think that big publishers (Ubisoft, EA, Blizzard) are doing the right choice in promoting their own stores and clients, why would they give money to an other publisher?

Valve just came first (it was very pionieristic, I still had 56k when Steam launched).

The reason I was a bit surprised was more because of the way GOG/CDP haven't seemed too encouraging of GOG keys being sold outside GOG itself. Maybe things have changed since that incident with Green Man Gaming.