Ueber: The deal seems to be some kind of a loyality service for steam customers, although it would be very nice to have the same deal on GOG, it does not make sense to have it here on GOG, if the OG wasn't available for purchase beforehand.
Its like complaining that you are not allowed use your miles, at a different airline.
Yes and no.
It makes no sense for GOG as the selling platform to offer a seller-side discount to buyers who own the original game on a different platform.
But, for a distributor/publisher-side loyalty discount it would make sense to not require the old purchase and the new to be on the same selling platform — whatever platform you choose, you're still loyal to the publisher/developer/distributor (if not the selling platform).
Things get more complicated in combined deals where both the distributor and the selling platform give up some of their profit margin in order to provide the discount.
Next, cross-platform checking to see if you own the game elsewhere would pose a technical problem. Which could be solved, of course, but then there's practicality.
Discount counts handed out on the basis of CD/serial keys by the distributor/publisher and redeemable on GOG would be an elegant solution, though then there would be the problem of preventing the recipient of such a code from also qualifying for the default discount and benefiting twice (e.g. in order to help a friend get the game cheaper).