Not that I agree or disagree - the idea of the GUI can be considered a logical extension of hypermedia and interactivity.

I think there is a difference between "GUI" in general and Xerox Park GUI specifically. Sketchpad and GRAIL clearly had GUIs of sorts, but Xerox developed UI concepts that were highly generalizable. And just because they became popular doesn't mean they were "obvious". Without Xerox Park all the computers we use right now could have drastically different interfaces.