Alexim: On GOG he's only listed as a developer, bureaucratically it's an abysmal difference.
idbeholdME: Oh, I see now. Hovering over the links on the "Company" line reveals that.
That's right, on the GOG store to the left of the slash you find the developer, to the right the publisher.
So if developer and publisher parted ways, obviously the game can no longer be sold even on GOG by the previous publisher. But if the new publisher wanted to (i.e. Ant.Karlov himself) he could get the game back on GOG. But he may have decided that it's not worth publishing the game on GOG, or simply no one has asked him what he wants to do.
ariaspi: If it's only about changing the publisher, then, hopefully, this will be only a temporary delisting.
It could, but it wouldn't be the first time the new publisher didn't care about getting the game back on GOG, so it could still be permanent.