As far as I know, the whole G&T programme was introduced to address the issues of higher-achieving children having their needs mostly ignored in school. For a very long time, there was the concept that bright children succeed no matter their circumstances. That is definitely not true. Bright children, if they're not given challenging material in school, just coast. And, worse than that, they often don't find out what it is to have to actively learn until much later, sometimes in their teens, when it's often a little bit too late.
The concept was good, but the execution was been very flawed, partly because the only way to measure how good a school was at identifying those children was through not much more than a box-ticking exercise. Good teachers have always been able to identify the needs of individual children and respond to that. But there was a time when the focus was on equality rather than differentiation.
Children of all levels of academic ability do develop at their individual rates, but there needed to be a means of permitting and encouraging schools to move away from the idea that all they needed to do was ensure that children had reached their milestones for that academic year. So, there could be a situation where a child enters Y1 able to read fluently, both fiction and non-fiction, and understood basic mathematical concepts, just because that's the kind of child they were, and also because of their home circumstances.
For too long, those children were just put into a standard educational programme which didn't address their needs. The G&T programme went some way to redressing the balance. A child who enters primary school being effective at reading/comprehension and good at arithmetic may well be undistinguishable some three years later from a child who entered primary school without that knowledge - but the difference is that the child who started school not knowing how to read or do arithmetic would have spent three years learning, the other child could well have spent three years just being bored.
I really should learn how to write short responses.