Depends on who you ask. I think agnosticism (in relation to theism) and atheism are distinct. I also don't see atheism as an ultimate truth claim saying we can know there is no god. Merely saying we there is no proof which would be necessary to believe in the particular gods that which others do claim to be real and relevant.
Agnosticism is a state of ambivalence. Possibly a transient ambivalence or possible a belief that we actually can't know either way. Atheism is a denial of a belief and bootstrapped understanding that at least theism can't possible be true.
Dawkin's made a nifty
7 point scale.
Many others on the web fall back to this other graph, which I disagree with. I won't post it because I think it's trash. But it puts Atheism and Theism on one axis, and agnosticism and gnosticism on the other axis. Each quadrant is a possible pairing of each atomic set. Gnostic Atheism is used to describe strong atheism, and agnostic is used to describe what I would just consider de-facto atheism. Agnostic Theism is really just deism, and Gnostic theism is theism.
The thing Gnostic Atheism should really describe de-facto atheism which is still only. "I can't possibly buy into theism, but I know I can't deny god. I will live under the assumption that there is no god or a deistic god is inconsequential to my life."