Gwynn attended Long Beach Polytechnic High School, which was a high-profile school in sports. He was a two-sport star in both basketball and baseball. In his final two years, his Jackrabbits baseball teams were a combined 3–25–2 in the league, while the basketball teams went 53–6 and twice reached the California Interscholastic Federation (CIF) Southern Section 4A championship game. Gwynn had considered quitting baseball as a senior to concentrate on basketball, but his mother talked him out of it. "She said it might be something down the road and that I might be sorry later if I didn’t play", recalled Gwynn. As a junior, he was the starting point guard on Poly's basketball team that went 30–1 and won the Southern Section 4A title. The final was played before over 10,000 fans at the Long Beach Arena, where Gwynn scored 10 points in the 69–50 victory over Buena of Ventura. His teammates included Michael Wiley, who became a professional player in the National Basketball Association (NBA). In his senior year, Poly was 23–7, and again advanced to the championship round in spite of finishing third in the Moore League. Their bid for a second straight title ended with a 57–50 loss to Pasadena. Gwynn averaged 10.3 points and totaled 178 assists for the season, and he was named to the All-CIF Southern Section Second Team. After the season, he switched to baseball, whose season was already well underway. Despite his shortened season, he earned first-team All-Southern Section honors after batting .563. Poly's baseball teams struggles taught him to remain focused and continue to execute and stay productive. Gwynn received scholarship offers to play college basketball, but none for college baseball. He also went unselected in the 1977 MLB draft, which Gwynn attributed to his limited playing time. He wanted to play both sports for Cal State Fullerton, which was okay by basketball coach Bobby Dye, but baseball coach Augie Garrido did not believe an athlete could handle both sports in college. San Diego State basketball coach Tim Vezie wanted Gwynn to commit to playing two years of basketball before playing baseball. Gwynn also had an offer from Texas Christian University (TCU). However, he was told that he would be their first black player in 30 years, which ruled them out from his consideration. He chose to attend SDSU, calling it "the best option I had".