Zimnel: It's time it starts to work the other way around then : )
joppo: Since that decision is in Valve's hands you should write to Valve and convince GabeN to provide Steam keys for Gog purchases.
Go ahead, we'll be waiting.
From what I understand, it is up to the publisher mostly. At least many publishers seem to be able to give away "free" Steam keys for their game, and Valve doesn't get a cut from them.
I recall e.g. the Postal 2 publisher suggested that those who are not happy with the GOG version (e.g. it not receiving the latest update + Paradise Lost DLC), should just contact them (the publisher) in order to get a free Steam key. I presume that offer is not valid anymore as now GOG version has received the missing parts...
So yeah, people wanting a free Steam key on top of their GOG version: your best bet is to contact the game publisher directly, and ask for the Steam key. I personally don't care for multiplayer so I'd be happy with the GOG version alone, as long as it has a meaningful and fully working single-player game.