Yes, I'm homeschooling 2. DD1 was on the G&T register at school and I took her out at 12; personally I think she's just pretty bright. She's 15 and has 7 GCSEs and is starting maths A level. Despite this, she doesn't really like academic work much so we don't spend too much time on it - she's into dance and skating. That's what she wants to use her brain for. As long as she is happy, I'm happy. I'm not going to push her into an academic career or to university just to satisfy my sense of pride. 
DD2 is 11. I think she's more what you'd call true G&T. We took her out of school at 8 because she was so far ahead academically the only way they could cope with her was to put her in with the 11 years olds; she was utterly miserable. She adores learning and soaks up every bit of information she can grasp. She finds everything sh does easy, from science to maths to music to ballet. She changes her mind about what she wants to do in life on a daily basis as she has so many things she loves. She's just taken Physics IGCSE and the LA asked on her last visit why she hadn't yet taken GCSE maths as she's working easily at that level.
It's a difficult one for me because I'm not sure what benefit there is in doing them so early. We're following GCSE courses for most things and she tends to extend herself into A level territory. But if we take them all now, what then? A levels at 13? I want her to have a childhood and there's going to be nowhere left to go. I conceded to the Physics IGCSE and she loved every minute - except that all the 16 year olds stared at her when she took it!
So instead I'm trying to broaden her out; she's following an astronomy course, learns lots of musical instruments, does lots of dancing and skating, draws and paints a lot, studies obscure subjects she has an interest in.
The nice thing is that most home ed groups are not based around academic ability. Emotionally she's actually quite young for her age and lots of her friends are around 9. They have no idea where she's at academically, much less care. This makes her life so much more normal than when she was at school.
I don't actually think G&T is all environment by the way, though I know that's a different issue. If anything, DD2 has been more neglected educationally than DD1 and I was more laid back with her from birth. Yet by a year old she was talking in full sentences. It was embarrassing taking her out because she looked tiny and young into the bargain and so everyone constantly stopped me to ask how old she was and why she was talking - as if I'd worked some kind of magic trick on her! I kind of knew from the start she was something a bit different.