As much as i agree with the gnostic atheist concept, i think the term atheist is perfectly sufficient and you are wrong when it comes to not being able to be sure about God's existence, we can't prove it to the theists because they shut off and flee into the imaginary world at some point in most cases, but we can be personally 100% sure.
I am 100% sure that their made up stories are what they are.. incoherent fairy tales from people back in the bronze age.. or even before that.
If there is some supernatural being then it is not something that we have any evidence for, we don't see it intervening and we don't see anything that clearly points into its direction at all.. meaning that we have no reason to believe it or even assume it exists, other than fear and imagination creating an illusion, and illusions are not a good guide for reality.
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We have proof that people were brainwashed for millenia to be kept illiterate slaves and tools and that they were by all means forced to stick to the idiotic obsolete teachings from ignorant inhumane slave drivers and alike, who later were replaced by delusional indoctrinated slave drivers and alike.
On the other hand "the unknown" is... well unknown, so I can't know for sure if there is something in the realm of the unknown which would even partially fit into the concept of "God", but we have plausible explanations for most mysteries that gave people a reason to make up and also "prove" their Gods, now we know for sure that "God" is not needed for such things, even for the beginning of time and the universe, but they didn't know any of those things and simply made stuff up to entertain their brains.
So for my part I can say "the man made construct that is God" doesn't exist in reality, it is a product of imagination and indoctrination, we know for sure that the stories are not true, for many reasons, contradictive concepts and lies included, make me 100% sure it is not existent in reality, period.
My logic and our knowledge makes me 100% sure that all of the uncountable made up Gods don't exist and never existed and that nobody promoting those imaginary beings has any proof or even a rational reason for their existence.
When we don't know something we should just say "we don't know" - fullstop until somebody comes up with something with proof or at least plausible evidence for whatever it is, starting with anything other than "faith" and "you are too stupid to understand" or "you will suffer imaginary pain in an imaginary place foreverandeverandever if you don't believe it and do what my book says".
Having all that said, i think i am safe to say i am a gnostic atheist when it comes to ANY God that the humans talk or even think about.
When it comes to "the unknown" i guess i am an agnostic atheist, but to keep it simple i call myself just atheist, like the people in the show, that usually does it :)
ps. great point with Santa.. i like to say "You are too old for fairy tales, reality works just fine, ask Santa."