Well, GD is 100% free, even for commercial purposes. No limited “demo” version or extended “Pro” version. That’s big advantage already. Another one is that events aren’t interpreted like in most other gamemakers I’ve seen, they are converted to c++ (native) and compiled or converted into very fast JS (html5) which add to speed of the engine.
The only limitation I can see is limited amount of extensions, but I’m pretty sure it’ll grow over time.
Anyway, if I’d have to summarize GD in one sentence, it’d be “The best mix between Clickteam’s products and Construct”.