I believe the "gifted" label means they are within the top 5% of their own school for a particular subject (or it can be identified from CAT tests). So definitely above average, but in a low performing school, I agree, they are not/may not be geniuses. Even in schools with a high proportion of FSM/PP students, you probably don't find that many students of this kind in the category.
I do think they are the type of students who should be able to get into a top 20 university/on to a competitive course if they fulfilled their potential (with the possible exception of Shakira as I'm not sure how academically able she was). I would think that if most of them came from a middle class home, they'd easily be able to aim for medicine/law/whatever they wanted.
However, given their backgrounds I think that if any of them achieve all 7/8/9 GCSEs, then it will be a real achievement for them, their families and their teachers.
I think at the age of 13 it's good to encourage them to aim high, and see a top university as the end goal, and then at sixth form it's where this needs to become more realistic/targeted (and this is where a lot of schools struggle as they're not careers/university applications specialists.