I think that the bottom line here is that most children are G&T in some way. There are some extraordinarily bright people around who were once children so there are obviously very, very bright children around too. But quite what the cut-off mark for being labelled thus is debatable.
Have SIL (and all her friends) who went to Cambridge who even at highly competitive grammar school was in the top 5% but she would never have called herself G&T! In fact she says that she only really remembers one girl at the school in the entirety of her time there who would really warrant such a label and she went up to Oxford a year early, played the harp to grade 8, studied all the time etc....But some people would probably regard all Oxbridge students as having been G&T children.
The problem is that children change and learn at different rates. Some children are very early readers but I am sure I have read research that indicates that there is no direct correlation twixt being an early and early advanced reader and future academic achievements.
Can think of several boys at DCs school who are probably lower than average attainment academically but already show the makings of excellent athletes and would be regarded as G&T in that area.
Isn't a lot of it subjective anyway? We have a DD who is bright but nothing outstanding in terms of literacy and numeracy but she has a very, very logical mind which seems precocious for a 6 year old. We think she displays a gift but because the national curriculum doesn't really encompass this it has never been flagged up. We could of course be deluded about her talents and because none of her teachers have picked up on it think we probably are. But are we?
Think some teachers do G&T and a lot don't. Some teachers are very keen to get talented children thus labelled but others (particularly the school hierarchy) aren't, it can cause great expectations which come to nothing for all concerned.
DW is a bit of a Tiger Mother but even she thinks the whole G&T thing is fraught with danger. And she avoids the G&T thread like the plague. One parent's G&T child would be another's decidedly average one and that is the humbling thing.