That's true. But still, as an example, some games are Steam redeemable even if they aren't made by Valve. I can understand if it's not possible, though.
Anything's possible, but to this point, there's been no indication that GOG is looking at this as an option, or even that they want to. I doubt the publishers would agree to it anyway. Steam is its own form of DRM, but the games here are DRM free, so giving the backup option would be seen by many people as a free game they can just give away. Hell, many people are looking at the backup Witcher games this way already.