Gnostic means you know, agnostic means you don't. Theist means you believe, atheist means you don't. A gnostic atheist "knows" there's no god and an agnostic theist thinks there's a god. In this case, GP is an agnostic atheist.
That depends on the formalism and the objective. If we accept agnostic priors then the most rational belief implied by those priors is atheism, via the same formalisms that give us Ockham's Razor. That belief is conditional but it is a very strong condition such that you would not expect someone that held agnostic priors to not also be operationally atheist for all practical purposes.
Similarly, most atheists seem to hold agnostic priors but fully realize the implications of that by calling themselves "atheists".
