rgnrk: They probably informed GOG about it. As to their GOG's customers, well, there aren't any at the moment.
Thing is, they don't have a platform to talk to their customers in GOG, while they have one in Steam. They don't really even have customers in GOG, nor an official post about the game where they could post into until the game is out.
They could have made an kickstarter update for all their backers, but maybe they thought it wasn't worth it for just one day. Obviously they did mentioned the one day delay in their official twitter.
You're absolutely right about the GOG part. In fact, my previous post was more like a general rant directed at no one in particular, than an attack to Puzzling Dream for this specific slip.
However, I still think they should have posted an update on Kickstarter, even if it was only to announce a one-day delay, as they obviously deemed it worthy of being announced on Steam. I don't use Steam or Twitter (or any other social network for that matter), so KS updates are my only source of information for any game I back, and I'm left completely in the dark if devs don't bother to post updates on the very platform that allowed them to make their game in the first place. Kinda rude, if you think about it.