I own the game and the former expansions on GOG.com, but the latest expansion has not yet appeared on the site. Any idea when it will available for purchase there?
I am taking that as a “Not gonna be on GOG”?
Anyone?
I wouldn’t be too sure about that if I were you. I’m somewhat familiar (my sixth year here, lol) with how Cliffski - the developer - plans things out, and he seldom reveals things until he’s 100% ready to do so. “Absence of evidence is not evidence of absence.” 
After he returns from his current trip in a couple of weeks, feel free to ask him directly.
It should be on there now!