I understand this is the norm elsewhere, but no, sorry - https://support.gog.com/hc/en-us/articles/212806085-Do-I-get-a-Steam-key-when-buying-a-game-from-GOG-

This isn't Humble Bundle where buying the game can net you a DRM-free and a Steam code. This is GOG, where all purchases are inherently DRM-free and run without Steam. If you are really concerned about it going through Steam, you can always add it as a non-Steam game.

In any case I encourage you to go with buying the game on GOG regardless of whether you'll get a Steam key or not. There's nothing to be lost.

if you, for some reasons, wish to own both drm-free and steam versions - I suggest you to buy games on humblebundle.
GoG is kinda niche and not so popular shop, so - adding such ability may kill it.
There are, actually, very few games, which supports linking from steam to GoG (e.g ones on GoG connect and some very rare with keys, like 2nd witcher), but not backward.

TLDR: no, you cant

Which was a promotion set up by GOG years ago to encourage avid Steam users who bought The Witcher 1/2 on Steam to try out the GOG platform. GOG/CDPR has and probably never will have any business incentive whatsoever to give away Steam keys for their games to GOG users. Keys are given for free to draw people from less advantageous platform A to more business advantageous platform B, and GOG would never benefit from driving people away from GOG to the Steam platform.

Some people might say for those games "well they lose out because I will buy the game on Steam and get free copies of the game on GOG because I can't do the reverse of that", but GOG/CDPR are well aware that a small number of people will do that and they're ok with it if they do because they ultimately still make money from those games if people buy them on Steam, just minus the Valve take. The free game promotions are not about the game however, they're about the platform itself and are intended to promote people to coming to use the GOG platform itself who either might not know it exists, might know about it but not have an account here, or any number of other reasons.

Also, the "good reasons" why a company does something have to be good reasons in the eyes of the company itself under metrics that they decide upon and not reasons the users decide are good for themselves. Promotion is not just about giving things away for free out of obligation or kindness, but about drawing people's attention to what you want to catch their attention for and in exchange giving them something in return for their consideration.

Normally the party in question that would do the promoting like this would be the publisher themselves, and publishers might give GOG keys to Steam customers on occasion (I've had that happen for a number of games), or Steam keys for GOG customers (I've had that happen for a few games as well such as some of the ArmA titles and a few others I forget off the top of my head), but that is the publisher doing it either directly or through GOG as a proxy (as with the ArmA games) but not GOG themselves. In the case of the CDPR games, CDPR is the developer and GOG is their sister company so they have a direct and indirect incentive to draw customers from the Steam platform to GOG with free copies of the given games, but no incentive to do the reverse.

Generally speaking you don't ever get free copies of a game on one platform if you buy it on another platform. Any cases where you can are rare exceptions that are tied to the purpose of promotion in some form or another even if it isn't publicized or directly advertised in that manner by the publisher of the game per se.

The easy way to think about it is "What benefit would the game publisher/developer get out of doing XYZ?" stated in terms that are relevant from the perspective of the game developer themselves rather than from how the end user sees it as beneficial to the end user. If one can't think of anything and has to concoct an unrealistic scheme to come up with an excuse, then it probably isn't something viable and probably wont happen a.k.a. "wishful thinking". :P

Not to mention that it has set itself up as a direct competitor to steam, and the little matter of DRM-free being a key policy. :P

nah, I dont like, what GoG slowly turns into either.
But, back to theme - bringing such ability isnt that easy and will require to change lots of license agremeents between GoG and devs/publishers. Its possible, yep, but, if there will be such feature someday, very few games will support it.
Coz again: GoG cant just add it once - everything belong to devs/publishers. I understand, that lot of newly added indies sells on humblebundle or itch together with steam keys, but, since GoG tries to deliver old games (as they position themself. "We sells indies only coz we need money to buy rights to sell old stuff", or something like this), there is no way to add it, without killing GoG, as it is right now. Feel free to call it "rebirth" or "rebranding", but thats it

Downrating legit questions from new users is a really good way to gain new members for the community. Yep.