It's important to remember that "atheist" and "theist" aren't very descriptive labels. It's perfectly possible to be an atheist and believe in heaven and hell (perhaps not the christian version, but some version), angels, demons, ghosts, magical crystals, psychics, etc.
Atheism describes one of two things:
1. A lack of belief in god/gods.
2. A belief there is no god/gods.
(The former being "weak" atheism and the latter being "strong" atheism.)
With this in mind, saying someone is an atheist really doesn't say much about them, just as saying someone is a theist really doesn't say much about them.
Also, gnosticism/agnosticism deals with knowledge, not belief, so gnosticism/agnosticism is perfectly compatible with theism/atheism.
If one is an agnostic weak atheist, one basically says "I don't believe there are any gods, but I can certainly be proved wrong. Ultimately, I don't know."
If one is an agnostic strong atheist, one basically says "I believe there are no gods, but I can certainly be proved wrong. Ultimately, I don't know."
I'm not sure if gnosticism is compatible with weak atheism. It has to be something like "I know that the answer to whether there are gods is unknowable, so I don't believe in gods."
If one is a gnostic strong atheist, one basically says "I know there are no gods."
If one is an agnostic theist, one basically says "I believe in god(s), but I can certainly be proved wrong. Ultimately, I don't know."
If one is a gnostic theist, one basically says "I know god(s) exist."