I'm sorry to do this, but the sale numbers on GOG don't justify the work going into keeping the gog version of the game up to date and make the upcoming DLC work here.
I don't want GOG users to have a worse version, so the right decision is to take it down, becuase parity can't be guaranteed.
Everyone who bought it here and wants the DLC or newer versions can get a Steam key from me by emailing me the Supraland receipt from GOG. And no: there is no DRM on Steam.
These kind of posts never make sense to me. If it is not viable to maintain games on both platforms, you should wait until the game is EOLd and then release the final version on GOG. No more maintenance, but still some passive income, even if miniscule.