I agree with notagain01 - you should not get caught up in the GHS hype.
As an observer of the GHS 11+ selection process over a number of years, you can generally spot a “GHS girl” in advance. Contrary to popular opinion, they are not always the very brightest girls. There are many girls who meet the academic bar required to get in. (A higher standard is required to get into the senior school than the junior school - there tends to be a preponderance of former junior school girls in the lower maths sets from Year 7).
At 11+, GHS is looking for girls who are competitive (hence the lacrosse section of the selection process) and confident in their abilities, who tend to excel at a sport, music or drama (rather than being good in a number of these areas) i.e. they are already specialists in one of these areas at the age of 10/11 and will be labelled as specialists in this area throughout their school career. In my experience, the school tends to prioritise selection of extroverts who can deliver the school the PR potential of county or national exposure in one of these areas over quieter, more introverted, academic girls who cannot.
There are many parents who make a conscious decision not to enter their girls for the 11+ exam at GHS as they know that the school would not suit their daughter temperamentally. If they have a more introverted daughter, who is going to work diligently come what may, they feel that the pressure of spending every day with a cohort of 100 or so competitive, bright extroverts is unnecessary. (Of course, if you feel that your daughter will coast without the competition from such girls, then GHS may well be the right school for your daughter!) Also, those parents who want their daughter to be exposed to a range of different interests throughout her school career, regardless of her initial ability, also tend to rule GHS out as its commitment to excellence tends to bar opportunities for girls to develop any nascent ability.
I think that, provided that you select the right school for your daughter, it will not matter whether she joins at Year 7 or Year 9 as she will find her place. Good luck!