Hi OP, I have the opposite to you :) my DS1 was born very very late August so just scraped into his current class & he is G&T but in many ways I wish he had been born a couple of weeks later & started school the following year.
He keeps up & excels at the academic side of things, he's on all of the top tables & is happy with the work. However he is always last in every race that he joins in with, he never wins on Sports Day, is never picked for football or sports teams as he just can't keep up. He's always the smallest & slowest. He is very emotionally immature, we walk to school with other classmates (they are in Year 6) & the other children walk sedately & talk together but my boy is the one bouncing around, shooting things & playing ninjas by himself. He's just not on their level socially or maturity wise & its heartbreaking at times as he's told me his classmates call him annoying. I hope that when he starts secondary next year he will find some likeminded friends.
But back to your OP, a lot of Reception is doing things that are not very academic, such as sitting on the carpet quietly for a time, toileting by themselves, putting on their coat & shoes independently, sharing, taking turns, being part of a team & its this stuff that the younger ones can struggle with. If your child is G&T then the school will pick it up & there will be something like a G&T club or the work will be differentiated a bit, starting a year early wouldn't change anything in this regard.
Find him a wonderful nursery then teach him to read & write if you like, take him to museums, art galleries, exhibitions, zoos & the like & get him interested in learning. Teach him to read for pleasure. If he shows interest in a subject then get to a library & get books out on it. If you teach him that learning is fun then he will be fine whatever class he's in, there's more to school than academics :)