According to that great and highly respected reference, Pub Quiz Help the collective noun for Gnus is an "Implausibility or Herd" of which I will ignore herd as it is too mundane :o
As the idea of being a Gnu Atheist is a bit of whimsy, I hope adding a bit of reason as to why it is appropriate to be referred to as an Implausibility of Gnu Atheists isn't too serious. Put simply:
1) Atheism exists as a counter to theism. If theism did not exist, there would be no atheists;
2) New Atheism exists because we are modern enough and have enough science behind us to know that any theism is extremely implausible, as there is no evidence to support it;
3) Gnu Atheism exists because New Atheists are not strident enough; and
4) The need to collect a disparate group of people with widely differing backgrounds together to unite and be strident against the ludicrous notion that adults that indulge in fairy stories and must be taken seriously is well, implausible.
So I put it that an Implausibility of Gnus is very apt, appropriate, and embraceable.