Aliciahammersmith excellent advice from Needmore. Having gone through this twice, once from a school that didn't prepare my DD and once from one that diid, my first, second and third bits of advice are; relax, don't allow the competitiveness and anxiety in the playground and beyond, often fuelled by Chinese whispers about what is required to get through the exams, which rarely arise from actual knowledge of what is required, to affect you and your DD, and relax. After the results came in for both my DDs cohorts it was clear that the schools know the qualities that enable a girl to succeed in their schools and irrespective of years of tutoring the girls got to the schools that suited their personalities and level of ability.
Also completely ignore the mindset that this is a first past the line race, based on attainment. It is one the tutors encourage because it is in their interests for panicked parents to think their DDs need preparing like prize racehorses. It was all the more frustrating when DD2 was prepared just in that way as a result of parental pressure at her Prep, it had little to do with educating, stimulating and challenging my DD intellectually. By January many were in a state of burn out (especially those with tutors on top), all enthusiasm squashed and it was all I could do to keep my DDs enthusiasm for intellectual challenge intact and her feet on the ground, and encourage her to feel that she should just go in there to show them who she was and what she could do. If they did not offer a place it was because it wasn't the right school for her, and there were plenty of other good schools. As it happened, though she was not particularly strong in tests of attainment at the Prep and regarded by them as average, my DD was offered places at all her choices including the one comparable to G&L her school thought was aspirational. This was much to the puzzlement of parents of girls who had been tutored and prepped and had been outperforming her in prep school exams who were not offered places, or put on the waiting list. She was actually above average once she got there and faced more challenging work.
If it were a first past the post race the schools would set straight forward tests of Maths and English based on the curriculum, no need for interviews, school reports, questions that explore different dimensions of ability. For G&L for instance if you look at the consortium papers there are questions on the maths paper that are more about applying what they know laterally, applying logic, those questions can keep parents thinking for days including a management consultant with an MBA and 2 maths graduates but it was fun 
but I suspect they learn a lot about the students from the way they approach those questions and their workings, regardless of whether they get the right answer. No amount of tutoring will prepare a student for them as they deliberately make them unpredictable The selection process is designed to explore your daughters ability and personality, as needmore says they want the brightest and best suited, not the most tutored. In fact the exams for the least selective schools are the most straightforward but they offer to a broader band. In all the schools once there you will find great diversity in terms of strengths and weaknesses. I have one DD who was top Maths set but threw a wobbly at writing a piece of creative writing and one who was bottom Maths set but loves to write, and is full of ideas about the books she reads (and still managed an A*in Maths at GCSE)
Another bit of evidence that supports the schools expertise in recruiting according to raw ability is that most end up supporting around 10% of their pupils to a diagnosis of Specific Learning Difficulties of one kind and another. They clearly can distinguish between shortcomings in attainment and accuracy, and ability. Freezing up in an English Comprehension would of course be a common issue for a pupil with a specific learning difficulty, and it might be worth exploring whether it is confidence and skills or a more fundamental issue with speed of reading, processing, and organising her ideas. Both my DDs are Dyslexic.
That said it is good to have them feel confident of their basics when they go in the exam room. With my older DD coming from a state school environment, so they had not even covered the full syllabus, I worked through a set of Maths workbooks that covered the curriculum keeping one step ahead in derusting my Maths but devised some questions that applied the concepts in unfamiliar ways. Whatever she struggled with we went back to basics and practised. We also did some logic puzzle books which she really enjoyed. For English we got a tutor because it wasn't so black and white in terms of a correct answers, and my DD isn't one to accept shades of grey from me after all I only have a related degree, what do I know but she was a teacher she had loved and it was a cosy session with orange juice and biscuits once a week for a term. She also helped her with exam technique. None of this started before September.
Couldn't agree more about reading widely and discussing with them what they read, not just because of the exams, I am proud my Scientist still devours fiction. 
For various reasons my DD1 was interviewed separately by the Head of one of the most selective schools, the one on a par with G&L. After she commented that "we probably learn the most from interview" and that in terms of overall accuracy and knowledge of the curriculum they set the bar fairly low with the intention of consolidating in Year 7 for those who come from schools who do not prepare. But they are looking for evidence that they have the qualities to best succeed at the school. It is potential they are looking for.
I hope this helps and reassures. Above all remember that in West London we live in a ridiculous bubble where these schools get differentiated on the smallest and most tenuous of factors. They really are all good schools and enable bright pupils to achieve their potential and get to the best universities. Schools like G&L get the results because they are more selective, but it should be about what schools feel right for your DD, that indefinable ethos and the culture of the girls, as much as facilities and exam results, it may be that they will be happiest and best achieve their individual potential at a less selective school.