Ask the schools about University destinations. Remember City has quite a small sixth form so absolute numbers going anywhere will be smaller.
At this level the best school for your child will be the one that suits them best. Journey, culture etc. There is a very bad habit in West London of parents ranking schools (and colleges and just about everything - with the assumption your child should be the best and have the best - exaggeration but at times almost offensive when kids then tell other kids that their school is ranked more highly than another's as if this means they are in some way superior). Some of these schools will be taking in a slightly wider academic range, but all will have a good proportion of clever kids and all should do well by them. They obviously each have a different feel, and girls probably pick up where they feel more at home. I know people who rave about G&L and how nurturing it is, whilst others prefer the more robust atmosphere of a co-ed.
All will also be capable of getting children into good American colleges. My guess is that G&L will have more experience, simply because of catchment and IB. And that given it is quite a different process, experience matters.
(G&L seems to host a number of talks by visiting US admissions people, to which pupils from other schools are invited. But other schools do as well.)
Sport is generally not high standard at G&L, though in fairness they have a reputation for encouraging participation. Latymer rowing could be of the right level. Perhaps their hockey if supplemented by some Club training. Latymer has strong swimming, and will soon have a 6 lane swimming pool, though they allow pupils from other schools to share some of the before and after school sessions. (SPGS, LEH and Putney High all play lacrosse which is a minority sport in the UK but popular with US colleges, and where it is is not too difficult to achieve a regional or national ranking.) Both schools do music well and have a joint orchestra. City don't have much of a sports reputation as far as I know, but have good music.
Both G&L and City regularly send girls to Westminster for sixth form. Two of the G&L girls we knew were American and both went on to good Ivy Leagues.