Indeed, the genesis of GEM was with Lee Jay Lorenzen who left Xerox to come to DR after he tried to pitch a light version of the Star's GUI that would run on PC-class hardware. So although there was obviously a lot copied from the Mac there was also a lot that came from his experience at Xerox.
Here's his presentation of his prototype, in 1982, quite a while before the Mac or Lisa were released:
Here's his presentation of his prototype, in 1982, quite a while before the Mac or Lisa were released:
https://www.youtube.com/watch?v=EMBGRZftS30