Guild rating for GvG is changed only by playing automatic rated matches and automated tournaments. The higher the opposing guild's rating compared to yours, the higher your reward for winning and the lower your penalty for losing.
Rating determines your rank on the guild.
The mechanics of rating changes
Guild Wars uses the Elo rating system to calculate ratings. In this system, the probability of winning is calculated for each guild or player, determined by the rating difference between the opponents before the match. The higher this probability of winning, the less rating is gained by actually winning the match.
With this probability, the actual rating change can be calculated by R' = R + K * (t – p) where R' is the new rating; R is the current rating; K is 5 during non-tournament play, 25 during daily tournament play, and 50 during monthly tournament play; t is 1 for a win or 0 for a loss; and p is the guild's win probability.