Please do not post unless you understand what I have to deal with as a parent of 2 profoundly gifted children.
Actually, you'll find Mumsnet is a public forum where anyone is permitted to post on any thread as long as they are not deliberately trolling.
I don't think that genuinely gifted children are uninteresting at all, by the way--that's not what I said in my post. I said that a group which is ostensibly "for gifted children" will attract a lot of parents of children who don't have any talents that stick out a mile but whose parents have merely pushed them very hard. I know from my own experience that when you start a "group" for a certain type of people you have in mind, very often it attracts people who may not take the purpose of the group seriously or are just not right for it.
I don't run a G&T group. I do run a couple of English-oriented groups for parents (I live overseas and support English as my child's minority language at home, so we do an English literacy Saturday school, and also a "getting together" group where we do things like museum visits and craft sessions in English).
The focus of such groups is of course very different from a G&T group, but my own experience is that the best way to start is to start off with a couple of friends, and then gradually expand by word of mouth. You are better off starting with a small, tight group and then gradually getting bigger, rather than putting the word out far and wide and ending up with a whole bunch of "latchers-on" who don't share the vision of the group but want to come and hang out anyway, or are very low-commital and disturb the harmony of the group by only turning up occasionally (which will also make it harder to establish a settled rota for taking care of organizational tasks). It is a good idea to have some kind of rota (formal or inform) for organizing events and meetings once you get beyond a certain size, otherwise all the burden will fall on one person and exhaustion will cause the group to fold.
Even if you find a group of like-minded people, it is extremely important with these kinds of groups to be respectful of people's different parenting styles, and have enough flexibility to ensure that everyone feels comfortable taking part. Please don't take this the wrong way, but your posting style does come across as pretty dogmatic and "my way or the high way." If you want a group to survive and thrive, respect and good listening skills are key, as is a sense of humor and humility.