english- your earlier remark : "I think GSs are a valuable part of the education system. But then, having been to one I would." is interesting.
I went to a GS, too (at the age of 10!) back when (1973!) they maybe did cater more for the 'brighter local DC', in that it had a catchment and there was one DD in my class who'd come from a private school. Even so, there were quite a few DDs there who shouldn't have been (I mean, 4 or 5 'O' levels?!) and we had a reasonable influx from the local girls' SM into our 6th form who had the requisite 7 'O' level, but no other local SM fed into our 6th form other than the odd one or two DDs. I should add that I never exchanged one word with my headmistress in 7 years, there, either! You only did if you were in big trouble or heading for Oxbridge.
The same school now has no catchment and is full to the gunwales with Prep schooled DDs and tutored-for-2+ years DDs. I know this as a fact.
But the thing is, I am opposed to GSs in their present form- more on this in a sec. When we arrived back in the UK from abroad, we inconvenienced ourselves to buy a house in a comp area; I suspect DS1 may have passed the modern 11+ for the boys' GS, but DS2 would not have, therefore I wasn't going to risk a SM for him.
About 'types' of GS: I am not opposed to the existence of a few, highly specialist schools for DCs -ahem- burdened with the SEN of being extraordinarily gifted but with off-beat social or inter-relational skills, 'oddball' but brilliant (in the same way as we have specialist schools for DC at the other end of the more or less NT spectrum). I'm thinking of, say, Winchester College where being 'just clever' (and rich!) isn't enough. You have to be off-beat, quirky, a bit 'other'. Mainstream public-school heading 'clever' go to Westminster and St Paul's (generalisation alert!). I can see a need for state school DC who'd fall into this 'quirky and very clever category to maybe be siphoned off into something other than the local comp, but as for the rest (and I mean, hey, if we're talking '23%' of the DC' as quoted, hardly a glittering, tiny, special minority, is it?!), a well run, properly resourced comp should be the destination of choice. I would add, entry to such a specialist school would not be via a single exam on one day. There would have to be years of evidence of this DC's particular abilities. It would be utterly 'untutorable' for.
Before you bleat 'but plenty of comps aren't 'well run and properly resourced', I'd counter 'nor are quite a few GSs!' (many of my teachers would have been slaughtered by a similar SM class!) but I'd also say, I wonder if 'the local comp' just might be 'better' if GSs didn't exist albeit miles away to cream off the brighter MC DC? Wouldn't such DC be an asset to their comps? I genuinely can't see why the clever and the less clever can't walk about the same campus in the same uniform together, separated into sets in each subject?
Finally- yes, example of 'good comps' do exist: In rural Australia in the 70s where if you wanted something other than the local comp, you boarded, they managed to run schools that sent DC to the best state universities and taught vocational well. DH was at one which I toured with him 5 years ago at a reunion. Well, there were flash, sparkly science labs, there was a professional quality theatre, there were industrial catering kitchens, there was a woodworking shed with bloody great gantry mounted band saws attached to the ceiling, there was a working farm where the sons of farmers learned how to birth cows, lay fencing straight and do farm accounting. This is a big school (2000+ DC) and DC do have journeys of 2 hours each way to get there (being rural Oz) but they didn't feel the need to cream off, into their own school, one group alone to be given special treatment because their ability lay in academic subjects alone!