When I was an undergrad (a year ahead of Gates, 72-76), Harvard had only "intro to programming" undergrad courses, so those of us CS (well, applied math) majors who arrived at college already competent at programming pretty much just took all grad CS courses for 4 years. The faculty were enlightened enough not to care that we weren't grad students.
Gates was certainly one of the brighter undergrads in those courses. I don't know if that makes him a "CS giant," but he was no slouch.
While you guys were coding away at Harvard, I was not yet able to properly focus my mind, and so took to running around the streets of Cambridge(port) in diapers instead ;-)
p.s. from the above one can assume I was either born in '72 or was taking far too much acid for my own good.
Gates was certainly one of the brighter undergrads in those courses. I don't know if that makes him a "CS giant," but he was no slouch.