I couldn't have responded any better myself.
Atheists have a lot to be angry about. In his article, Gellman writes:
Perhaps their atheism was the result of the tragic death of a loved one, or an angry degrading sermon, or an insensitive eulogy, or an unfeeling castigation of lifestyle choices or perhaps something even worse.
This is an implication that atheists all have some sort of emotional or psychological issues otherwise they wouldn't be atheists. Then Gellman wonders why atheists are mad at him.
Gellman is unable to comprehend that most atheists are atheist because they don't think that the religions make any sense. Someone who is "atheist" because he is mad at God isn't really an atheist at all.