Anyone knows how to activate The Witcher on Steam if you bought it from GOG? I remember that they said you could do that during their big announcement a few weeks ago, when they introduced the mac version on Steam. They said that they're just introducing the mac version, and if you already have the game on GOG, then you don't need to re-buy it on Steam, but you can just activate it there.

Anyone knows how to do that?

Only if you have the Steam Mac version can you activate a Steam PC version. You can't activate if you only have a GOG version.