Sadly I did ask the developer and he never responded.
I'm hoping it was because he was too busy and not because he's been rejected by GOG.
IronArcturus: Ugh, really? Why do game devs use the registry to store
save game data? What's wrong with the game's own directory?
Normally I'd agree with you and others here about the saving thing, but in this one instance I am completely on Toby Fox's side.
When you actually play Undertale the way it uses it's save data makes sense. The dev is trying to say something, he wants choices you've made to stay with you and I can't say more without spoiling anything.
lepke1979: Stil, I'd be willing to bet that GOG has dismissed it as being "too niche" as it starts to use Steam as its guideline for acceptance.
Seriously, If I find out GOG rejected Undertale I will actually consider stopping buying from here.