Well I don't know I've never got that label despite getting 9a*s yet a girl who got A'sand b's is defined as g and t at my school?
It varies a lot between schools. Before GCSEs I was G&T for subjects which certainly weren't my best, and it was just a bit of a school exercise. At college, they took the top 35 to 40 students based on GCSEs for their G&T programme, which I think worked out as 3 or more A*s at GCSE.
This topic vexes me. G&T is so unnecessary - it means absolutely nothing in the end. Students should get the best grades that they are capable of without all these silly titles.