Gnu atheists is a term coined--or, alternately, coyned--by one Jerry Coyne, as a pun on New Atheists. Probably, as with much of what happens on Web 2.0, Jerry didn't so much wish to create a new meme as publish a funny graphic, but it has since been noted that, since 'New atheist' has always arguably been a bit of a misnomer, Jerry's coinage may have easily as much utility.
Consequently, there's even a badge you can attach to your pages, now. As well there should be.