a child locally gained a place at a specific primary with a good reputation for g&t because the school they offered her was used to dealing with a large catchment of children with lower than average attainment on entry (ie their main effort was put into raising standards to average by the end of KS1.) i don't think they had a g&t policy even - it just didn't feature... the child in question was v gifted at maths too i think - scarily so, much different to the average more able child like mine lol.
as others have said, you would need to prove that the school he has been allocated would not be suitable. fwiw this does occasionally happen - the ht of our catchment primary told me that the school was not suitable for any of my children (i was looking for a yr r and a yr2 place at the time, and then a yr r place again two years later), so it may be possible that the school he has been allocated isn't suitable.
i think you are on fairly dicey ground for juniors though tbh - all of the junior schools should be able to offer differentiated maths to a 7yo who is working 2, 3 or 4 years above average by virtue of their age range alone - unless you could prove that he was working at yr 7 or above and that the school you want him to go to has specific links to a secondary that would need to be utilised for your son's education to be supported, then a g&t appeal might not really be worthwhile.
if that is the case (ie he is working above the KS2 curriculum and therefore it is access to KS that is the deal, not a junior school curriculum - he may have sat the end of KS2 maths paper this year?) then you need to make that clear - you might win on the grounds that your preferred school is better able to deal with g&t juniors, rather than someone who fits into an existing junior school curriculum.
that was waffley, eh?
fwiw, we decided not to appeal the junior school place for my 'g&t' dd1 lol (she was working at 4s and creeping into lower 5 but well within the junior ability range)- but she got in before the start of term from the waiting list.
helpfully, in reserve we hold dd2's wheelchair and the junior school in question is single level as opposed to the school she was allocated which was 4 storeys with no lift) but we didn't need to mention that
hth