Gabbert was born in Ballwin, Missouri, USA and went to Parkway West High School, where he played high school football. In his sophomore season, he threw for about 1,100 yards and 11 touchdowns as a first-year starter. As a junior, he had 1,523 yards and 20 scores and also added 458 yards and 8 TDs rushing. He threw 623 passing yards and five touchdowns as a senior, despite an injured foot that limited him to four games. He was considered one of the best players entering college at his position.