Adamdos: If you Right-click on the Witcher EE in your Steam library,then pick "View CD Key",you'll get a code that can be redeemed on GOG.com.If you haven't used the code yourself,you could gift that.
HoangMarcel: Can this method be applied with other game? I have Psychonauts on my steam library and want to add it to my GOG account.
Unfortunately,it doesn't work for Psychonauts.It might get added to GOG Connect at some point,Double Fine already gave some of their games away through Connect.If you got Psychonauts from Humble Bundle,when it was free in September,there should be a DRM-free download in your Humble library.
As far as I know,the only Steam games that can be redeemed on GOG.com are The Witcher 1+2 and the STALKER trilogy.
Some EA Games on Steam can be redeemed on Origin using the View CD Key method.Copy and paste the key into the 'Redeem Product Code" screen.