Good hunting! I'm with you, we need to know why GOG rejected this game, of was developer decision to not publish it here
because his other games doesn't sold well and he don't wanna bother with the effort to create a different build for GOG and give the proper maintenance (a thing we know happened before with other developers).
It wasn't. Judging by the tweet above, Zach had intended GOG to be the
primary DRM-free distributor for the game and had to look for an alternative ASAP at release. Whatever the disagreement was, it wasn't "nah I don't want to maintain a build here". This
is why the situation is so doubleplusungood: if GOG can't resolve a disagreement over the amazing masterpiece that is Opus Magnum with Zach, how can we hope they'll do anything about Discworld or NOLF?