Strong advocate for Tormead, which you mentioned above.
GHS is obviously a good choice, but don't discount the other Girls' school in Guildford, especially as academically it is excellent (obviously not GHS), but better than 'pretty much' everywhere else in the immediate vicinity, and it has a solid reputation for pastoral care with down to earth girls. That's not to say GHS doesn't have these attributes too, which I'm sure it does, but it still might not be the right fit.
Your best bet is to go see them both, as that will ultimately inform your decision - the worse mistake anyone could make is to pick a school just down to 'academics', so go see what they both offer on-top.
Worth noting on the Tormead front, RGS prep school is literally next door, making drop off a piece of p***, lol. Lots of DD/DS siblings are at Tormead and RGS Prep. Also on the Tormead front, with your little one, who will be too young for RGS prep, Tormead have just acquired Rydes Hill School, which has a co-ed nursery, Reception, Yr 1 & 2, and from 2026 will be growing again, adding Year 3 all the way through to Year 6, as the current Yr 2 progresses upwards.
So if you want Co-Ed for your DS, then that's a solid option.. There is also multiple child discount between Tormead and Rydes Hill, which applies across the two schools, plus a shuttle bus starting in September, so you can drop everyone off at Tormead and the bus will take the kids across to Rydes Hill and bring them back after school.
Anyway - GHS good, RGS good, Tormead good, Rydes Hill good (especially now it has Tormead backing behind it) - so you've got some great options. I'd call, have some tours or do all their open days.
PS. when planning your strategy, just remember that all of the above (apart from Rydes Hill - which is still growing), are very popular to get into, so start thinking about it early. I only know about Tormead, but it is full in a lot of Prep School years and in Yr 7 again this year (Sep 2025). I've also heard of friends' DS's touring from other local schools that are not doing so well, who have been told places in the Senior Section are in short supply too and some have been turned away sadly.
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GHS and RGS are always oversubscribed at 11+!!
Good luck!