If I am not mistaken, Zach stated in a Reddit post that the game will be available on GOG (and possibly Itch) once it is out of Early Access on Steam. Also, the game is practically in its final form, with only a few finishing touches to be added (including one more minigame), based on the feedback from Early Access users (you can find this info either on Reddit or Steam - I don't remember exactly). So this should take no more than two months, like with other Zachtronics releases.