Edit: I didn't know that this thread was exclusively related to Tony Gwynn. I thought it was about quirky sports facts which is why I included the Walter Payton info.
Walter Payton retired from pro football as the all time rushing leader, which everyone knew. What a lot of people don't know is that he also retired with the all times receptions record and receiving yards record for running backs. He also threw 8 touchdowns to no interceptions over his career and he was the Bears backup punter, backup field goal kicker, backup kick returner and emergency backup quarterback. He was also considered among the best blocking backs in NFL history and there is an actual Lawrence Taylor story about a block Payton threw against him.