|The colors are arbitrary, just the standard colors from Microsoft Word.
Now, the original diagram had parts of the extremes cut off. The original author said he cut off the extremes because there are very few people that hold the extreme positions. The ideas are NOT from current word usage and more from the traditional concepts behind the actual words. The word gnostic in the diagram is in no way related to the term given to the heretical view called gnosticism (from ancient Greek) except to use the same Greek term, "γνῶσις gnōsis, knowledge" (from Wikipedia). Also, this table is not an attempt to make new terms. There is an important distinction to be made though, philosophically. The discussion about the existence or non-existence of God often asks, "On whom is the burden of proof?" Well, the burden of proof is on the person that is making the claim.
If a gnostic-atheist stands up and says, "There is no god, and I know it." That person bears the burden of proving that claim. Also, the strict (original meaning of the term) agnostic, if he/she stands up and claims, "We cannot know whether or not god does or does not exist." That too is a claim and must be defended. Likewise, if the gnostic-theist says, "I know there is a God." Then he/she must prove it. Now, this is all philosophically speaking, and thinking like a debate wherein there are rules by which one must abide. If, you're simply seeking a worldview that makes sense, there aren't really "rules," it becomes more about what evidences and arguments to which one is willing to listen.
I would argue that no truly honest thinking person would ever be on the top half of this diagram. Here's the problem though, people talk "big talk" and in books, lectures, blogs, etc. People talk like they know one way or another when really all the "gnostic" positions are lying to themselves and everyone else. Here's something I heard a while back from Ravi Zacharias and lately I read from an apologist on Facebook, and though I don't know the exact words I'll try to capture it the way it comes to my mind:
To absolutely affirm a negative, is to claim infinite knowledge. To affirm the negative that there is no God, is to claim infinite knowledge that there is no being that has infinite knowledge.I assume you see the contradiction there, though some don't see how affirming a negative is claiming infinite knowledge. Here's one way to think about it, I affirm the negative that there are no such thing as unicorns (an affirmation of the negative). That means I know for certain that there are no unicorns anywhere or at any time. Ironically, unicorns are often used as examples of things that everyone knows don't exist. There's an interesting conundrum with that claim. Take the initial premise that there are an infinite number of universes, a multiverse. *This is a common claim which runs counter to the claim that the universe is uniquely fine-tuned for the existence of our galaxy and life itself. So, here we are with an infinite number of universes, and within at least one of those universes a unicorn must exist. If you don't think so, then you are misunderstanding the concept of infinite!
Back to the diagram. There are so many positions on this continuum. There's rarely anyone on the far extremes, but there are lots of people that sit somewhere in the middle. People that don't think there's a god, but allow that there might be one. People that think there might be a god, but aren't sure about his/her/its existence. There are people that don't care and sit right in the middle. Let's not redefine terms, let's stick to these ideas. Language is vague and that makes things difficult. People might say, "I'm an atheist," and they're really saying they're an agnostic-atheist. Same with theists, some claim that God exists, and if they're being honest they mean they're pretty sure that God exists, but they're not sure. I often hear internet-atheists claim that atheists aren't making claims at all, which is pure ignorance. If you say you don't believe in god because there's no evidence you're not being honest with yourself or those you're dealing with. There is plenty of evidence, the problem isn't the lack of evidence, it's what you will accept as evidence. Will you accept a simple straightforward argument? If not, if you try to explain it away with some kind of untestable theory (like the aforementioned multiverse) then it's not about a lack of evidence, it's about outright dismissal of evidence.
Edit: After a short conversation with a coworker there may be an edit in order for affirming a negative. Here's a possible exception to the idea that affirming a negative requires infinite knowledge. If I make the claim that there is no such thing as a square circle, I do not think it requires infinite knowledge to affirm a philosophically contradictory claim. So, to say, there is no such thing as a square circle or a married bachelor, is affirming an already established contradiction and does not require infinite knowledge. Here's an interesting point that this route would take an atheist should he/she go down the route of philosophical arguments. Many atheists, including John Loftus (though to be fair, he only speaks out against the philosophy of religion and not philosophy itself), are against philosophy, and for good reason too. Looking at this site, the logical arguments against theism are just weak. I'm only an elementary philosopher and I can see through these arguments like glass. Also, in all my discussions with atheists both face-to-face and online, they have always appealed to science. In fact, all I've ever heard from atheists, including in their books, is the mantra "science, science, science." Philosophy is on God's side, in a manner of speaking.