9m GP more doesn't tell us kitten... Question is about relative GP and number of players, since registered GP is used, not guild GP. How many players did each side have? What was your registered GP?
Just some quick math... Say you are a 50m guild and they are a 59m guild, that works out to about 180k/player. Not a huge difference. So in that scenario you had what, 1 more player than them?
Edit: 9m over 50 players is always 180k, what I am saying is that if you took in 50 players and they only took in 25, each of their players would have to have, on average, 2-4x your players' average GP