I have 2 daughters at AGGS.
I think there is a lot of mythology over how hard it is at AGGS. Yes, they work you relatively hard and have high expectations, but it really doesn't appear to be a hot house. Some girls do struggle and the school appoints mentors from the senior girls who meet with the struggler and support them in whatever way is necessary. Lots of the subject groups run informal, drop-in helpdesk sessions at lunchtimes where you can go with particular problems.
I think you should worry more if you thought your daughter came from a prep school and had been intensively tutored for years than if she comes from a state school and had one hour of prep a week. The state schoolers tend to thrive - once you get the reduced class sizes, remove the boys and any disruptive primary school element, they do very well. It's more the prep girls, who are used to smaller classes and very directed learning who struggle.
Girls do get 90% but don't fall into the trap of thinking that's what they all get. My Y11 daughter has just finished her mock GCSEs and has come home with tales of Maths results distributed evenly all the way down to the 20% level