If you want comparison of the three schools (on characteristics which actually matter, as PP have said somewhere someone has misguided you on exam boards):
GHS is probably the most 'academic', gets the highest results and has the reputation as being repeatedly in the top 10 girls' schools in the UK for GCSE or A Level. It is very driven to let girls achieve and encourage them to be strong, dynamic women in whatever field. Lots of fun and care too though and the girls are full of character. It had had some great building work done recently and is up to date in terms of wellbeing offering.
St Cats is the next most academic. Much more a country school (with lots of girls boarding in sixth form). It more pushes young ladies, manners and is a bit more jolly hockey sticks, house spirit and traditional. But it is still up there in terms of academics. Equally, has had some impressive building work done pushes sciences and stem recently.
Tormead is a bit of both. Town school but the head there is quite old fashioned. Results aren't as strong but it provides a nurturing sixth form.
My DDs, back in the day, were at GHS. I would strongly consider st cats over it now but unsure what I'd eventually pick. Both my dds loved it up until sixth form where they just muddled through to the end to be honest. They gained a lot from GHS' proximity to RGS boys' though!! My dds have very solid friends from GHS which they still see as they enter their 30s! Most of them live in London and work in the city. All fairly career orientated types balancing marriage and children with their ambitions. A few of their friends are quite insecure, high-maintenance anxious types but that is more personality and true of any school (also they went through ghs years ago when there was less focus on the whole person and encouraging resilience and confidence etc). I've heard similar of girls from all sorts of schools though so probably shouldn't link it to GHS. Both schools are wonderful and offer a lot. I would imagine your dd would get the same results and uni offers from either. It comes down to what environment you want- town or country.