Hmm, I think I know what might have happened. The OP here says the delisting is due to expiring licencing rights.
If you look at the game on Steam, you'll see it's both developed and published by Gambrinous.
If you look at the details on the GOG page (
here's google's cached version of the page from a few days ago) you'll see it was published by Versus Evil.
So my guess is that the original publishing deal that Gambrinous worked out with Versus Evil has expired (the game came out in July 2015, so it sounds like there was a 5 year contract). It looks like the developer has transferred or at least speedily renegotiated the contract at Steam, hence them already being listed as the publisher there. Unfortunately, for one reason or another they haven't done so with GOG. I'm hoping this is just a case of them (or GOG) acting too slowly and that the game will be back before long. The alternative is I guess they might have decided they can't be bothered to keep the game here. I certainly hope that's not the case, but it might be.
Here's the dev's twitter by the way:
https://twitter.com/gambrinous I tweeted to ask why the game was removed, but I didn't get a reply yet. Still, it probably can't hurt for others to ask about the disappearance. Just be nice and don't pester! Getting a message from multiple people is probably fine, but getting multiple messages from one person is probably annoying.