I am getting conflicting information here... the demo reminds you to save the game at the end so you'll be able to pick up in the full game where you left off. But other official channels say that this will not be possible
Which is true?
(Sorry, GOG forum won't let me link - search for the phrase ""a number of things that would be flagged in the save file" on Google and you'll find it)

I think saves will NOT be compatible.
And this is probably what you were trying to post - http://steamcommunity.com/app/264520/discussions/1/540737414496590204/

Maybe the Steam demo can't save games in a format usable by the full game (because it only has Chapter 2...) and that is what they are talking about.

Yea, I think it might be an issue that some items you might have from chapter 1 aren't present in the "chapter 2 only" demo, so you end up not being able to actually continue the game.

Yeah the Steam demo has a different setup than the GOG demo - since it is a carefully wrapped up Chapter 2, the saves wouldn't be compatible with the full game. We use special variables to keep the demo properly contained in that chapter and functioning nicely. Since the GOG demo just starts at the beginning of the game, there's no issue with that. There should even be a message that comes up when you try to begin Chapter 3 telling you to save your game before quitting so you can pick back up at that point when the full game releases next week.