Oh Gates ABSOLUTELY ripped off Mac OS... because Gates was given early access to the Mac in order to develop Word for it. The only reason why the lawsuit between Microsoft and Apple failed over this fact was because a contract based around the development of windows 1.0 sucked and was in Microsofts favor, and Apple couldn't provide more explicit copyright infringement beyond look and feel which back then was not enough to advance a case like that.
Ironically Xerox tried to do the same thing to BOTH companies out of regret around the agreements they made surrounding the GUI in general, and also failed at it in part because they waited too long to even submit a copyright claim, and because Jobs had actual documents FROM Xerox agreeing to the use of technologies around the GUI in exchange for stock from Apple, because at the time Xerox saw absolutely no future in the GUI or the mouse.