A two-time conference champion at The Barstow School, Gillen was named MVP, All-Conference, and All-District all four years in high school. She was also named Conference Player of the Year and selected to the All-Star Team her senior year. She achieved 1,000 kills during her junior year and had a career-best 30 kills in a game her senior year. In addition to her accolades at the prep level, she was named to the 2018 Prep Volleyball Senior Aces List.
Jillian is the daughter of John and Kady Gillen, and has two older brothers, Jay and Jared. Her father played basketball at UMKC, her mother played volleyball at Morehead State, and Jared plays baseball at Swarthmore College. Gillen intends to major in business.