To what Matiasdu said (hola paisano), which relates to this particular (and quite obvious) point:
Steam keys shouldn't be given away for free if you aren't also offering the same deal (i.e., give the game away for free) to Steam customers.
I only add that it can only be through an economic deal involving CDPR and Valve that something like the OP asks could be done (I didn't need to read the above info to know that). And who would pay for the costs of that deal? The consumer.
I'm not paying for anyone to have the game on both platforms, that's for sure. If a game offers such a thing (2 for 1 on platforms), and you do not want to use that second platform, you are paying extra for others to do so with your money.
It gets tiresome to have to spell out things like these.
Going to the particulars of the case of having "free" keys on Steam for redeeming on GOG and viceversa, that would certainly destroy GOG sales of the game on large areas of the world, and maybe even their userbase in the long run, for the same reason Valve would agree to it: regional pricing. In this case it would be GOG that would not accept.