I have these games on steam with a key. Possible to use this key for a GOG DRM-free version?

Kind regards
Davor

No, GoG is in no way related to Steam. Therefore they're economic competitors, which is why you will have to buy the games again if you want to have the GoG version.
However the games are identically in their content. So the choice is yours ;)

You are right in that Stalker (most likely) isn't redeemable on GOG but the competitor argument thingy is rather meaningless with all distributors except GOG selling Steam keys by now (even Origin) and Witcher 1 and 2 having been redeemable on GOG even if bought from Steam. True, the Witcher games were an exception since they are owned by the same company as GOG but it shows that it's really up to the specific arrangements between the rightholder and the distributor, chances are that eventually there's gonna be a bunch of games redeemable in both places. Also there was at least one game developed and published by a third party sold in retail that was redeemable on GOG, maybe even both GOG and Steam.

The Witcher series is an exception since Red CD Project and GoG are related.
If a publisher and/or developer decides to give one the opportunity to redeem their CD-Keys on GoG - irrespective of if there is a Steam redeem option or not - than thats great. However you can be sure that than there is an sort of arrangment between GoG and those guys and this arrangement will include some talk about money ;)
GoG isn't a samatarian good-game-world place. The guys here a doing this because they want to be an awesome platform for DRM-free games, but also because they're making their money with it. And when people can redeem there Steam/Retail Keys here without a monetary compensation money will be lost ;)
It is simple as that....
EDIT: Key resellers also paying money for them ;) Therefore Steam takes profit from this aswell, as would GoG. But I haven't seen a GoG reseller yet. Probably because there are most likely no buying conditions for mass-purchases...

You actually repeated a lot of the stuff that I wrote. :P
Just sayin' - it's not impossible for a Steam game to be redeemable here and the same logic that applies to games sold somewhere else than Steam that are redeemable there could apply to GOG. That it hasn't happened yet (and maybe never will with anything else than CDP's games is a different story).

Yes you can redeem your cd keys from steam to gog for these 3 games. It's pointless though unless you just want the extra goodies. I redeemed them and it was easy also don't get steam keys and cd keys through steam mixed up

The Steam codes for Clear Sky and Call of Pripyat worked fine for me, but the serial for Shadow of Chernobyl, extracted from the registry after installing it, did not work. Some work, some do not. I still have the game at the end of the day on one or both of the services.

My key for Shadow of Chernobyl also doesn't work.

Go into Steam, select Stalker, click on CD Key on the right, paste it into the redeem form.
Worked for me

Yes, you can reclaim all three Stalker games with keys from Steam. I just did it. The only problem is the first game, Shadow. You need to run the game to get the CD key. The steps are as follows.

1. Launch Shadow
2. From the Main Menu click on Network Game
3. Notice CD key with all the XXXXs next to it in the top right.
4. Click on the XXXXs
5. Your CD key is now revealed.
6. Go to GOG and reclaim using your CD key.

I just did it. Worked like a charm. Love the reclaim feature. Will move more of my library to GOG as I can. It is by far my favorite digital games service.

That doesn't work for me :(

Hmmmm. Worked fine for me. Maybe it's something wonky like GOG doesn't have a complete list of all the Stalker keys, so some of them don't work. Maybe if you contacted support with the key you're trying they might be able to tell you what's up.

My retail Shadow of Chernobyl key just fine, while Clear Sky Steam key didn't work.

Some steam CD Keys classify as retail (they'll say so when you redeem them there) - though it also depends on what GOG got given in its database. For all I know they've got the serial numbers from a shipment of melons cassified as retails keys for Witcher 3. O____o