Interesting the graph puts a little asterix at Gnostic Atheist as being 'stronger than strong atheism, since it includes a claim of knowledge' yet doesn't do so at Gnostic Theist which also makes claim of knowledge.
'Strong atheist' is probably a term that whatever the source of this graph is had previously mentioned or defined. 'Weak' and 'strong' atheism is a dichotomy I've heard talked about in the past, whereas 'weak' and 'strong' theism for whatever reason isn't.
I suppose if pressed I would go with some combination of what both Als said (although I do understand the back-derivation of the term 'gnostic' in this context), but, like the political matrix, this chart makes presuppositions about the nature of the question that I don't necessarily agree with.