I have looked at the NAGC several times too and found it quite useful, however I never joined as I am not sure what they could do to help beyond provision of resources that seem readily available on the internet, but someone on here might enlighten me and maybe I will join.
I think there is a lot of anxiety on parents' behalf when their children start school and see that they are not learning at the pace they are expecting, being unchallenged in certain aspects and this is echoed by posts on here.
I think the label gifted does not help as it focuses on that rather than the fact that children have different needs.
My older DD1 has always been expectional in everything she does, interestingly now she is in Y2 and she is only slightly above average having being totally ignored at school all of reception and Y1, whilst she excels at everything she turns her hand at outside of school, whether academic or sports or languages or music. Every single provider she has had dealings with has approached us to say what a gifted/talented child she is - however school seem to actively try and put her down all the time.
You could easily argue that she is "levelling out" but I know deep down that her ability to learn remains very high, is just that she has not had the opportunity to learn and the ones provided at home are not enough to make up for lost time at school. Also in a way, I have preferred to focus her spare time on activities she enjoys, and where she is free to excel without having to be embarassed/derided for it.
So I am always a bit wary of the levelling out argument in this context.
Having said all that, I have seen with my DS1 for example, that children and boys in particular, can mature later so it is not necessarily the case that a child at 4 is the same child later on - my DS1 is a classic example of this, he seems to be blossoming all the time and making huge leaps in certain areas.
I would say that battling with school is a no win no win especially in the early years, it is very hard to change a mindset and indeed the teachers have few resources and are spread thinly so will view a parent who goes in with certain expectations as pushy and unreasonable in the end.
This of course is only my experience.