ZivilSword: Thanks for the hint. I can’t think of reasons why the developer won’t use the early access program on GOG, but at least the release seems to be available this year.
(Also I am repeatedly disappointed about the communication culture of many service providers – in this case GOG because they could at least put some text explaining the situation, maybe with the developer’s words.)
Because for whatever reasons small developers still think they need to use a Publisher, and those Publishers are morons who think every pirated copy is a lost sale. GoG games are easy to pirate, that's the only reason I can see to not release on GoG at the same time as Steam.