After browsing the C2 forum history that seemed the overall consensus.
Just a few weeks ago we had no manual. If you're looking at old posts you're probably reading the "there's no manual" consensus, but now it's largely in place and I think that makes the situation much better!
GM has a significantly different UI and workflow. The 'event' system is completely different (and very basic compared to Construct 2's events), and C2 has no scripting short of using the SDK. However, I suppose you could carry over the general principles of logic, coding and game engines, which is of course universally applicable!
I'd recommend just using the materials available for Construct 2 to get to grips with it - obviously we'd love stuff like printed books about it, but we're just not big enough yet for anyone to take that on board.