In a certain sense we don't know anything. I could be a brain in a vat connected to the matrix, you might not exist at all, but are merely a figment of my imagination, we could both just be a part of someone's dream. We can never be sure that some craziness like this isn't happening, in that sense we don't know anything. But this definition of knowledge isn't useful, and it's not really the way we use the word except when we talk about god or perhaps during drunk philosophical conversations (seriously, I throw great parties).
If I hold a pencil out at eye level and let go, it is possible that a glitch in the matrix will make it hover, it is possible that an alien has a tractor beam on it that will pull it to the ceiling when I let go, it is possible that god will perform a miracle and make it fly against the wall instead of toward the floor. In this sense, there are incredibly low probability things that could happen and we can't say with 100% certainty what will happen next, and yet we are all comfortable saying that we know the pencil will simply fall to the ground. Why is it that in this example everyone is perfectly happy to say they know for sure what will happen to the pencil, but when we talk about god people are so quick to fall into this "we can't know anything for sure" nonsense?
The only other wrinkle here is what is meant by a God. In these discussions people will often back up to a deist God who created the universe and then was hands off. They have a point there, in relation to that God it makes sense to be an agnostic atheist. But let's be honest, that is not the God we are usually talking about. We are talking about Yahweh, a God who knows everything, has absolute power, cares about us, and interferes in our lives. This God doesn't exist, it just doesn't make any sense.
I think for any reasonable definition of knowledge, I can safely say that I know Yahweh doesn't exist. Am I going to start calling myself a gnostic atheist? I don't know, I imagine it would cause more problems than it would solve even though I think it is accurate. I can hear it now, people would say that I now have accepted the burden of proof and have to prove that God doesn't exist. But let's think back to our examples of Santa and the easter bunny. We are all perfectly comfortable saying we know they don't exist. We are gnostic a-santa-ists, we are gnostic a-bunists, where is our burden of proof there? I imagine we are all happy with the lack of evidence where we would expect evidence to be. Why isn't that enough for yahweh?