The G&T children at one school I know do activities like visiting the Guardian and then making their own newspaper. At another they are involved in the BBC School Report, so get the chance to interview famous people.
The kids who are struggling with Maths, at the first school have "shopping" trips to town. At the second spent a morning at Ikea practising maths.
If you are musical you get involved in choir tours, concerts etc. If you are good at sport you get out of school early to travel to places.
If you have behavioural issues, you get sports camps etc. (some organised by the local police).
These kind of activities are on top of any general enrichment open to all children (activity days, school trips, drama troupes coming in, visits by authors, visits to the theatre).
Of course if you are G&T and especially good at Science, the school may just offer you an Art camp. Which is a bit like a chocolate teapot. Or as my niece was offered a Saturday maths course, which not only too up Saturday's but was full of boys and so she didn't go for long.
IMO G&T is useful, but it should make the teachers differentiate more. Thinking in their planning how to stretch the able. And definitely not using the G&T as extra teachers for the less able (all the time). This was something I hated at school, as well as being intolerant and impatient so awful at helping anyhow.
I do still resent my junior school BTW because when they had to pick an inter-school quiz team they didn't pick me (on fairness issues). Although the only time I was picked first for teams was for quizzes. My one chance to shine, and they didn't let me on ideological grounds.
I'm not saying there shouldn't be any special activities for the G&T, although what is G&T in one school can be middle of the road in another (another debating point). But I'd like a few random extras for those firmly in the middle too. (Actually I have heard teachers discussing this too.)
Now please don't flame me too much