I do make mental notes, lots of them, it's just that some don't seem to stick as well as others.
NB this is from UDK 02/2012
GFxMoviePlayer which extends Object (the base class for everything in UDK) is pretty much the viewport and is permitted only one gfx movie which is set in it's MovieInfo variable, this all makes perfect sense now.
GFxObject also extends from Object, but this time it's constructor uses the 'within' term, which I can only guess (for now as I have no idea) that within the context of there actually being a GFxMoviePlayer and therefore a GFxMovie that a GFxObject can exist.
class GFxObject extends Object within GFxMoviePlayer
It's also worth noting that in the comments above this constructor, someone just as confused by all this, took it upon themselves to straighten things out with this gem:
GFxObject is GFxValue in Scaleform code
GFxValue the fundamental datatype in scaleform code for a MovieClip, now that I can understand