Gillman fought back from a two hole deficit at lunch and a three hole margin through 26 holes to eventually regain the take lead at the 33rd then won the last to win 2 up.
The 16 year old Gillman, from Austin in Texas, is heading for a stretch in collegiate golf at the University of Alabama in two years time but this victory in the female game's most significant amateur championship elevates her status in female golf.
Gillman had earlier beaten the only Australian to make the match play phase Su Hyun Oh in the quarter finals on Friday.
Want video tips delivered straight to your inbox? Subscribe to iseekgolf.com newsletters.