I don't know who's fault it is anymore, GOG has since refused to take responsibility. But that's why I'm leaving this message here. As it currently goes, GOG users have to shell twice as much money to upgrade to the Royal Edition, compared to Steam.
There is no game difference between the 2 stores, so I see no reason why GOG users are being treated as second class citizens and paying more. I have no intention of buying the expansion (much as I wish to), as I would only be supporting the person behind these decisions.