I'm so excited knowing contacting GoG Support would let me transfer my Steam Wallet to GoG Wallet also. Thank you for the hint.

Steam, they first said you had to have bought the new franchise pack, which included both games and all dlc's.
They couldn't recognize and auto add the DE when you owned both games and all dlc instead of the franchise pack.

They eventually solved it by offering a webpage and e-mailing them.
Might've changed, haven't kept up with it.

I see, thank you!

That's different - that's related to the way subscriptions are polled in the Steam Store itself for vendors on the Steam Store. The amount of data that can be passed through the Steam API to external websites is different and more restricted.

Or was there some deal that I missed that let people switch their Darksiders 2 Steam version to GOG?

You didn't miss any such deal.

Nuh I was just talking of the deathnitive edition on Steam being given for free to those who owned darksiders 2 bundle on Steam. I remember it because back then some people were angry that even though they bought game at launch or when it was new and bought dlcs one by one didn't get free upgrade.

Well, I wouldn't have missed it if I needed it anyway - I only had the DLCs on GOG, not on Steam, so I only have the DE on GOG as well :)

I believe the point is to make you buy the DLC from GOG...

I doubt that's the point. it's a limitation of Steam's api since Valve are extremely lazy about doing anything other than adding new features nobody asked for.

It'd be nice if they could find a way to work around this limitation though. I have GalCiv III's founder edition so I get this stuff for free on Steam but I avoid using Steam for games whenever possible anymore. I'd much rather them on a better service such as Gog but there's no point in me paying a bunch of money to get all that again.
According to the Connect page it sounds like Gog has to get the devs on board with it before they'll make it available through Connect.

Which has nothing to do with Steam. It's the devs/publishers that must agree to it.

Doesnt work for me at all.

Can do what I want I always get error: NO ELIGIBLE GAMES FOUND

But it has. Those devs/publishers clearly prefer good ol' Steam for main partner over GOG and show clearly their preference of it over us, through this whole fiasco. I don't like being shown that i am not wanted as a customer, being reminded that i am a second class gamer, or being forced to use a shop i don't like and i don't agree with its rules and licenses. If you like that, however, it's your choice/opinion and it must be both heard and respected.

Is your Steam profile set to Public? If so, just give it more time. This Connect thing has Steam's API slammed right now.
Again, nothing to do with Steam. How is it Steam's fault that the devs/publishers prefer Steam? Dude, your hatred of Steam is blinding you to where the fault lies here.