Game extras don't seem to match on GOG website and Galaxy and there have been some added material to artworks and avatars. Was there supposed to be a notification if your games get additional extras (not updates) or am I imagining things?

I haven't seen any notification. Soundtracks were updated a few days ago before the launch and I knew it only from forums. Other people seemed didn't have a clue either. I could tell other extras were updated only by comparing size in Galaxy and in my extras folder.