When you apply to take the test, you state which schools you'd like to share the results with (so you would put Pates, although probably worth keeping your options open and sharing with all the Gloucs grammars). After the test is marked (centrally), the schools rank all the students who shared their results with them. You will then get informed what rank your child came for each school. Each school has 150 places, and will offer in strict rank order.
So if you have a super-bright child who ranks in the top 150 at Pates, you will get told they're in the top 150 and you have a guaranteed offer at that school. If they rank below 150, it's a waiting game till 1 March, when you'll find out if you get an offer.
This year, the lowest ranked child that Pates made an offer to 1 March was ranked 190. Its very likely that some children ranked below that will get an offer between now and 1 September.
How far down the ranking they go depends on how many children don't take up their offers - e.g. due to moving out of the area, choosing a different grammar (or even comp), or going private.
As a super selective, Pates only offer down to 170-210. The other grammars can offer as far down as the 750th ranked child or so.
Pates is the only grammar which has a pass mark. If your child ranks below the pass mark, they won't get an offer or be allowed to sit on the waiting list). (This year, the pass mark was achieved by the 244th ranked child). In theory, this means Pates could take less than 150 children, but I don't think this happens a lot/ever.