Just a quick question, hopefully someone can answer it. I recently bought S.T.A.L.K.E.R: Shadow of Chernobyl on Steam. While checking for mods, I came across this site so I decide to test CD key. I got the Steam CD key to work, but now I'm wondering if it would be possible to refund the game on Steam and play it through GOG. My main concern though is that if I refund the game on Steam, would it void the CD key and not work on GOG?

Even though https://www.gog.com/reclaim doesn't mention it specifically, the page for https://www.gog.com/connect covers this - If you're really doubtful of it, you could always ask GOG directly.

If you refund on Steam, you have no right to the GOG version, as you no longer own the game.
You can either refund on Steam, and buy the GOG version instead, or keep the Steam game, and play the GOG version.

I'm not a fan of Single Player games using Store Clients, so I would buy the GOG version.
Which I already do for games I own on Steam, and can't refund.
So find connect a waste of time, and essentially a bad idea, for the following reasons.

What's to stop someone, buying the Steam gamre, and only playing the GOG one.
Then when game is finished, refunding the Steam version, using the played under two hours refund polocy,
There's nothing to stop them keeping the GOG offline installer as well, for a single player game.
Any multiplayer may need a key for accessing devs/publishers servers, but not to play single player.

I'll stick to buying the GOG version, and removing the Steam one, from my library.
I don't want it, if I have the GOG version, I've done that for about 40 games so far.