Steam don't accept the key :/

Well the reason is perhaps in fact, this is GOG.com and no Steam. Download your game here. There is no connection to steam bro.

Okay after collecting myself laughing so hard.

This is GOG.com a DRM-free store, it has no connection to steam at all.

I felt a great disturbance in the Force...

Click on the link in the email you received from GOG to redeem the game. It will be added to your GOG account and you can download the installer from your GOG library.

In the initial promotion, GOG said they were going to also give Steam keys for the game.

They lied.

Welcome to GOG vs Steam competition filled with disinfo.

Gog never told me that and i don't know how you got that impression. Can you prove they lied?

Link? Or just talking out of your...

I'm fairly certain GOG Galaxy is intended as Gogs equivalent response to STEAM.

GOG and steam are not in alliance either. When we put out Rise of the Triad 2013, there did not appear to be any connection then, nor can I see one now. Galaxy is supposed to be GOGS equivalent of STEAM except without DRM etc.