Look, brycie, it's really quite simple. In GS areas, the 'top' x per cent on one day in November when they are 10 years old sit some tests. A few week/months later they get told whether they are GS material or not, and then their parents apply for their school places on the basis for that.
If they 'pass' they might still not get a place at the GS. If they 'fail', even by one mark, they don't get a GS place (unless, as in my area, falling rolls means that GS are taking kids they wouldnt have done a few years ago).
If they all go to the same school, the ones with the highest marks will be in the top sets with others of the same ilk. The ones on the borderline could go into those top sets immediately, or they could go into a slightly lower one and have the opportunity to move up/down, and the ones that didnt have an 11+ chance could actually suddenly 'get' the whole thing and fly high. Meanwhile, the ones who struggle are getting the support they need at their level.
In a fully comp system, the ones that are brilliant at maths but crap at french get the support/extension they need in each subject. And everyone can see that people's talents are enormously varied, and that being top set in maths does not necessarily mean that you are god's gift to the world, while being crap at maths and brilliant at dance/drama/textiles means you have a talent which is being rewarded - while at the same time, you are being supported in your weaker subjects, and have the opportunity to be stretched as you get stronger.
It's not that us parents at sec mods want the GS genii (is that a word?) it's that we don't want children put into little boxes when they are 10/11 years old.