First, my congratulations to your DD.
To answer your question...
G&T is a scheme started by the government circa 1999. Purpose: to find, cater for and encourage the more able in academic subjects and sports. At first the top 5% of pupils were meant to be identified and put on the register. After a few years the government's own review found that G&T was a shambles and wasn't achieving anything. So in a brilliant move to demonstrate that they were serious about G&T, they raised the 5% to 10% and considered the problem solved.
Today, this is how it works: The government emphasises how important the more able children are, how these children are our future etc. Very serious noises are made about how they will be provided for, how many millions will be devoted to them and how they will shine.
The government instructs local authorities as to what is expected of the LAs including that up to 10% of pupils must go on. LAs have to do something. So they do. They draw up guidelines, advice, paperwork. They do studies, collect stats. And they pass the buck. They ask individual schools to maintain a G&T register, have a G&T policy, appoint a G&T coordinator, decide how they are going to identify which pupils go on the G&T etc.
Schools then further water it down. They draw up a policy saying that it's entirely left to teachers to choose. Or that a child has to be 3 years above his age. Or 10. Whatever they want. Policy done. Box ticked. They'll go on to say that they'll aim to do this and attempt to do that. If they use words like "ensure" it will be in the form "we'll ensure we try...". All the LA's requirements are now met.
Then the implementation. The school finds someone morally/ideologically oppossed to the G&T principle and makes them the G&T coord. The G&T coord privately discourages teachers from putting anyone on the G&T. Or puts them on for the annual school census and then takes them off (that's why some coords don't tell the parents the kids have been put on). Even those that are on end up getting zero extra resources/attention.
So many millions end up getting spent. The children who are meant to benefit get nothing at all.
Schools vary, of course, in how they approach G&T so I wish you all the best. You'll be well armed by getting a copy of your LA's guidelines on G&T and the school's policy in the matter.