GD as it's most often displayed as is Goal Differential. What does this mean and how is it used? We'll break that down for you below.
Goal Differential is most often used as a statistical tie breaker between two teams tied on a league table (standings). If two or more (2+) teams had the same number of total points (awarded based on the results of matches), the team with the highest Goal Differential will be awarded the highest position. This is also used as the primary tie-breaker in tournaments with a group stage, such as the FIFA World Cup, CONCACAF Gold Cup, etc.