Sorry have been busy over the last few weeks/months so haven't been able to post
To answer the questions in the thread.
Schools have to decide their criteria for selecting who goes into G&T. Parents can get a copy of the doc from the school. Most schools are better at drawing up the doc than following it.
It's not just academic, also has sections for sport, music etc. as smee says
Whether a school chooses the max of 10% or 2.3% or those 2 years ahead of peers, it's got to be based on ability and potential rather than achievement.
Schools are expected to inform parents but there's no rule about it so crap schools often withold that information as a PR measure rather than including parents in the catering for and stimulating these children. They can't have a "policy" to not tell - report them to the LA if they do.
The G&T scheme, contrary to the beliefs of many Mumsnet posters who get often get their knickers in a twist about elitism, isn't desgined to benefit a few selected children. It creates a benchmark for provision that is meant to flow down so the entire school benefits.
The register is fluid - children can go on and off depending on intake and the speed their own abilities develop.
It's NOT only form filling. Schools are required to show HOW they are catering for the pupils with the highest potential (just like they are required to show how they are catering for SEN). Some schools are pathetic at this, a few are good.