His last name is Gates, because Gates is his family's name. His last name Gates comes from his ancestors.
Bill Gates' last name is Gates
Gates McFadden's birth name is McFadden, Cheryl Gates.
Andy Gates's birth name is Andrew Gates Freeman.
Antonio Gates's birth name is Antonio D. Gates.
Daryl Gates's birth name is Daryl Francis Gates.
Harvey Gates's birth name is Harvey Harris Gates.
Jake Gates's birth name is Jacob Gabriel Gates.
Lionel Gates's birth name is Lionel Theron Gates.
Phyllis Gates's birth name is Phyllis Lucille Gates.
Victoria Gates's birth name is Victoria R Gates.
Bill Gates' full name is William Henry Gates III