This is very exciting. Really a breakthrough for GM.
box2D is closely integrated, in the GUI you can add physics properties to objects and rooms, you will be able to add vertex shapes to sprites in the GUI, though you can use rectangles and circles as well.
Through GML code you can do all of the above as well and you can handle joints and motors.
Question: is the GML code "native box2D" syntax? Or are you creating GML functions to implement it? Either way's fine... just curious how you're doing it.