I'm familiar with G&T in 3 districts:

In the first, students are leveled for math placement by testing them during the first week of the school year. They are placed between 1 and 3 levels up for math--in this case, they're expected to be at least 1 year advanced, which was a little rough for some kids, actually. Otherwise, very little differentiation.

In the 2nd, all students worked 1 grade level up in all subjects, no differentiation.

In our new district, the program itself is less regimented and classes are split 4-5-6th grades all in 1 room, so definitely expected to be a range of abilities. Used to be lots of project-based learning and math was independent & at-your-own-pace.