(A)theism and (a)gnosticism aren't mutually exclusive labels; (A)theism is a matter of belief and (a)gnosticism is a matter of knowledge. If you believe in a god, but it's a matter of faith rather than knowledge (you don't meet your god for Sunday golf, can't spot it with a telescope, find traces of it in the geological record or construct a mathematical proof of it's existence), you're an agnostic theist; you believe in a god, thus you're a theist, but you don't know that your god exists, thus you're agnostic (lacking knowledge). If you believe in a god, and you know through personal experience that it exists (it talks back to you, you feel it's presence, the geological evidence for the biblical flood is overwhelming), then you're a gnostic theist.
I'm an agnostic atheist (a.k.a weak atheist) about most gods, since I lack a belief in them but I don't know that they don't exist, and I'm a gnostic atheist (a.k.a strong atheist) about certain gods, like the Judeo-Christian god, since I lack a belief in it and I know that it doesn't exist, at least in the form that's been put forth by the mainstream, since it's contradictory (the whole 'omnipotent/omnipresent/omnibenevolent: pick two' problem, or the square circle problem).