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.