Hi Gabrielle
The thing is, Pewley is on the other side of town from both GC and GA. Having said that, Surrey CC does not operate a fixed catchment for these schools, so your children may get in, depending on how many people apply in that year and where they live.
There are other schools in Guildford. St Peters is a very good Catholic comprehensive, although if you are not Catholic your chances of getting in are slim. Christs College and Kings College are the other state schools in the area, not as highly regarded as GC and GA but I think still good. Kings College operates in partnership with GA, not sure how this is working out as my children have now left GA.
If you can afford it there are some really good independent schools in and around Guildford, several in close proximity to the Pewley area.
I would also advise keeping an open mind regarding primary schools. Holy Trinity has a good reputation but there are other schools just as good, which might enable you to live closer to your preferred secondary school to enhance your chance of getting in. I am thinking of Boxgrove, St Thomas', Burpham, Guildford Grove.
Good luck with your move, Guildford is a great place to live and raise children!