Since proof that the deity of any major religion exists, or doesn't exist, is, by definition, impossible, that affirmative belief there is not God is exactly as much an act of faith as the belief there is.
Well, no. There is such a thing as burden of proof. "There is a god" is an extraordinary claim that comes with a burden of proof. "There is no god" is as much an act of faith as "there is no santa", and does not come with a burden of proof, because it is not an extraordinary claim, rather a plausible one, since nothing observable in this world has "there is a god" as its most likely or simplest explanation.
I think we can be both atheist and agnostic: we'll never know if there is a god (while alive), I don't really care about the answer, but I also find it very unlikely and so I don't believe in it, while accepting that I might be wrong. In other words, atheism is not necessarily a gnosis, depending on how you define it.