Ellis began working in theatre at 10, alternating the titular role in the original cast of ''James & the Giant Peach'' in 2010 at Goodspeed Musicals by Tim McDonald & Tony winners Benj Pasek, & Justin Paul, under Tony-winning director Graciela Danielle. The following year, he played Marty in Michael Kooman & Chris Dimond's musical ''Dani Girl'' in its development under Scott Schwartz's direction at NYC’s Cherry Lane Theatre - one the first real children to play the role, subsequently playing Colin in Jane Staab & Susan Kosoff's ''The Secret Garden'' which debuted at Boston's Wheelock Theatre. From 2011-2012, he played Prince Alexander in "Rumpelstiltskin" by Steve Marzullo at the Workshop Theater in NYC to critical acclaim. He won the 2012 "Best Performance by a Young Actor" IRNE Award for his run as Prince Chulalongkorn in ''The King & I'' at North Shore Music Theatre, played John in Tony-winner Phyllis Newman’s adaptation of her late husband (Adolph Green)'s "Peter Pan" musical for American Opera Project, & workshopped the musical adaptation of Dr. Seuss's ''The 5,000 Fingers of Dr. T." In 2013, he was known as "the kid with the big belt" on Seth Rudetsky's Sirius XM show ''Seth Speaks'' & appeared in a 4-person revue for the City of New York titled ''Welcome to NYC'' with Tony winner LaChanze & nominee Robert Cuccioli. In his early years, Gage was a boy soprano performing & recording new opera by Glen Roven, Tony winner Stephen Schwartz, & more, providing vocals for The Metropolitan Opera's featured contemporary works & singing roles at Boston Lyric Opera. 

​

Ellis graduated from St. Mark’s School in 2017 where he made Independent School League history by single handedly creating the institution’s first Musical Theatre program in over 20 years which is now part of the curriculum. 

​

Following high school, Ellis was a swing on the US tour of "The FlyBoys" (as seen on Britain's Got Talent) with the Glenn Miller Orchestra, starred in “Happily” (NYMF/GLSEN), ''Imaginary Boy,'' (Kennedy Center/Tourette Association of America), & "Niagara," Ryan McCurdy & Lauren Widner's musical (Carriage House Theatre) 4 months before Widner's death. In 2021, he led the star-studded cast of the renowned world premiere of Danny K. Bernstein's "Far From Canterbury'' at The Barnstormers Theatre & appeared in "Princess Clara Of Loisaida" at Bay Street Theater. In 2022, Ellis's performance as Jack in "Into The Woods" (BroadwayWorld Award nominated) at Philadelphia's Arden Theatre Company garnered rave reviews - that year, he was also seen in the development of the new musicals “Noble Family” (directed by 2-time Tony winner Richard Maltby), "The Gospel According To Heather" (by Tony nominee Paul Gordon), & "Poupelle of Chimney Town" (opening at Signature Theatre NYC in 2024). In 2023, he was seen in "The Monkey King" at Queens Theatre & workshopped the musical "Runaway Home" with Oscar nominee Abigail Breslin & Melissa Gilbert.

Most recently, Ellis most played Gabe in the original Taiwan company of the Tony-winning "Next To Normal" at the National Taichung Theater, praised by audiences for his re-interpretation of the role. In 2024, he will serve as a principal swing in the original off-Broadway cast of "White Rose."

​

Other favorite credits include Prospect Theatre, Huntington Theatre, The York Theatre Company, & Passage Theatre. Ellis is an experienced Jazz singer/arranger, regularly providing entertainment for Bloomberg Philanthropies, the Asian Cultural Symphony Orchestra, private clubs, and receptions. 

​

Ellis is active in community and equity work in the theatre industry & is a part of organizations such as the Theatre Industry Political Action Community & National Asian Artists Project. He is a proud eating disorder recovery influencer & is determined to change the entertainment industry’s conversations around beauty standards and appearance discrimination.