An Ice Hockey Wiki article.

Goals against average (GAA) is a statistic used in ice hockey that is the mean of goals allowed per game by a goaltender or goalie. It is calculated by dividing the number of minutes played by 60 minutes (one game) then dividing the goals against by that answer (the calculation does not count empty net goals or shootout goals).

Example: If you allowed 4 goals in 180 minutes your GAA is 1.33 (180 divided 60 = 3. 4 divided by 3 = 1.33).

Another way is to take the amount of goals against, multiply that by 60 minutes (one game), and then divide by the amount of minutes played. It is typically given to two decimal places.

Example: If you allowed 4 goals in 180 minutes your GAA is 1.33 (4 times 60 = 240. 240 divided by 180 = 1.33).

The result, both ways, suggests that for every full game you play, you allow 1.33 goals.

Since the statistic is highly dependent on the team playing in front of a goalie, save percentage is usually considered a more accurate measure of a goaltender's skill.