"Gnu Atheists is a pun on the label New Atheists that quickly took on a life of its own. As a pun, it is equivalent in meaning to New Atheists, and the two can be substituted for each other anywhere they occur."
Not to be confused with GNU atheism, where one doesn't worship, but uses the software anyway.
You don't have to worship, but are contractually obligated to preach any changes to the flock.
I gather that this is a particular brand of atheism which is obsessive to an obnoxious degree at trying to smash everyone else's religious beliefs.