Post 109 (12-4) led 7-3 before scoring nine runs in the sixth inning - helped by three walks, two hit batters and an error - to break the game open. Gaffney scored in every inning.
Gaffney pitcher Dustin Byars pitched a complete game, seven-hitter, allowing just two earned runs and recording two strikeouts.
Union (3-11) will host Inman at 7 tonight and again on Tuesday for its final games. Gaffney is second in League VI and heads to the playoffs.