In terms of sports, loads are done - hockey, netball, cricket, football, gymnastics, rounders, basketball, volleyball. Fixtures are regularly arranged throughout the seasons.
On the GCSE to A level pont, I don't think this is unusual at all. Sometimes people just reach the maximum of their ability level (or interest) at GCSE and just cannot make the same leap at A level. I was at school with several girls who all scored far higher grades than me at O level but just couldn't acheive the same at A level. Many of these did poorly or did not even complete their A levels. The jump from GCSE to A level is huge, hence why in many key subjects ie maths, the GCSE is done early in year 10 so work can start on the A/S work.
I agree on the pushed down the science/maths route by parents. Many of DD's classmates have said what they are going to be ie doctors, from year 7 and one of the mum's told me in year 7 exactly what her DD's year 9 options were going to be!!!
My experience hasn't been one of unhappiness, as I said, DD loves it. The teaching is mixed but overall really good. Would I send DD there again if I had a choice? Definitely yes. Is it the best school around with the best teachers etc? Probably not, but like all schools there are good and bad bits. Bit TGS definitely has way way more good bits than bad bits in my view and its the best school for DD. It may not be the right school for your DD and it certainly isn't for DD2, she won't be going there. As with considering all schools, keep and open mind and be prepared to be surprised.
For example, I visited LEH expecting to be blown away by it and absolutely love it but came away feeling disappointed and let down. It is a beautiful school with fantastic facilities and amazing resutls but there was something about it I didn't warm to and I can't put my finger on it. You may well visit TGS and come away with the same feeling. Keep an open mind and consider what's right for your DD (and I don't just mean in terms of academic considerations - personality and character, to me, have a big part to play in your decision making).