Gallup played two seasons at Butler Community College. As a freshman in 2014, he had 780 receiving yards and 11 touchdowns. In 2015, he played in only four games due to an injury and had nine receptions for 74 yards and one touchdown.
Gallup transferred to Colorado State University in 2016. In his first season he played in all 13 games, recording 76 receptions for 1,272 yards and 14 touchdowns. In 2017, Gallup was a finalist for the Fred Biletnikoff Award.