Gwynn Jr., who has played parts of eight seasons in the big leagues and spent time with the Phillies last year, signed with the Nationals on March 2. The 32-year-old has spent the season in Syracuse, batting .228, but he came through in a big way on Sunday.
VIDEO: Gwynn hits walk-off single
In the bottom of the 10th inning, Kevin Keyes led off with a single and moved to second on Rick Hague's sacrifice bunt. That set up Gwynn Jr., who hit a ground ball into right field off Buffalo left-hander Andrew Albers, scoring Keyes for the game-winning run.
Only five days earlier, the baseball community observed the one-year anniversary of Gwynn's death, which came after a battle with salivary gland cancer. In 2014, Father's Day fell on June 15, the day before Gwynn passed away. Gwynn Jr., who was with the Phillies at the time, recently told MLB.com how he called his dad that day but wasn't able to talk with him.
"I told my mom to just wish him a happy Father's Day and that we'd talk some other time when he could talk," Gwynn Jr. recalled. "And I never got that chance. He passed the next day."
Gwynn, a .338 lifetime hitter, won a record eight National League batting titles and was selected to the Hall of Fame in 2007. "Mr. Padre" played his entire 20-year career in San Diego, where Gwynn Jr. also played from 2009-10.
Andrew Simon is a reporter for MLB.com. Follow him on Twitter @AndrewSimonMLB. This story was not subject to the approval of Major League Baseball or its clubs.