Gilbert edged the first session with a bright start, claiming the first two frames.
Willams produces stunning comeback to reach semi-finals
Williams steadied himself and looked at his commanding best when he scored the first century of the match with a 121 break.
However, Gilbert instantly improved upon that, with a break of 142 - the joint highest of the tournament - and took a 5-4 lead into the second session.
Gilbert then powered into a healthy lead in the second session, claiming three consecutive frames, winning a scrappy first 68-58 before two breaks of 95 took him to 8-4.
Gilbert's was on the cusp on a first ranking title, but little did he know what Williams had in store.
Williams stayed in contention, fighting back to 8-5 before an 84 break put Gibert just one frame away from victory.
Williams then went onto win five consecutive frames to secure the win.
Williams sizzles to coast into World Open quarter-finals as Selby crashes out