And yeah, I've never actually seen someone hold to the idea of gnostic theism. I've only ever heard of it as a strawman from fundamentalist theists, or as a philosophical point, an example of a possible stance if not probable stance.
It's rarely consciously presented as a belief, but you get plenty of "Religion is so irrational and therefore wrong!" people, like the Oatmeal (at least according to that second comic). The belief that all religions (and especially Christianity) are wrong is very widespread - and it falls, as far as I can tell, into the category you call (or tried to call ) gnostic atheism.
Just because the major world religions are irrational doesn't mean there isn't a god, though.
It's simple to show that a certain doctrine is definitely wrong, when it contradicts itself. That's what most "atheists" do, since yeah - You can't absolutely prove that there is no god. The skeptic doesn't have to belief anything, just point out the inconsistencies.
A skeptic sees that all the major religions are either inconsistent or have no observable impact on reality (and thus are as meaningless as the invisible space unicorn theory), or both. This means that, despite centuries of all of humanity searching for the divine, we haven't found a testable one. It's a reasonable (but not provable) conclusion that there *isn't* a god who actually affects the world at all. No belief involved.