Gilder is the son of Admon Gilder Sr. and Paula Gilder, who is a breast cancer survivor. He began playing basketball at eight years old. He attended
James Madison High School in
Dallas, Texas, where he participated in basketball, cross country, and track and field. As a freshman, he averaged 15 points per game and earned District 11 3A freshman of the year honor. As a sophomore, Gilder averaged 22 points per game and led the team to a state title, being named District MVP. He led Madison to another state title as a junior and was named tournament MVP, averaging 26 points per game. He scored a career-high 43 points as a junior against
Dallas Carville High School. As a senior, Gilder averaged 30.8 points, 9.6 rebounds, 4.8 assists, 3.4 steals and 1.4 blocks per game. He led Madison to a 25–8 record and a Class 4A state semifinal finish, earning recognition as the 2015 11-4A District MVP. Gilder was named Gatorade Texas Player of the Year. Gilder was considered a four-star
recruit by
247Sports,
ESPN,
Rivals, and
Scout. He was a member of ESPN's Top 100 in the Class of 2015 and was ranked by ESPN as the sixth-best prospect in Texas as well as the No. 17 shooting guard. In November 2014, Gilder signed with
Texas A&M over offers from
Baylor,
Oklahoma State and
SMU. ==College career==