Agnosticism is a separate category. Agnostic means "without knowledge."

For example, you can be an agnostic atheist i.e. "I don't know if there's a god(s) or not, and I don't accept that there is."

You can be an agnostic theist i.e. "I don't know if there's a god(s) or not, but I accept that there are."

What you are speaking about is the anti-theist, the one that accepts the negative proposition, i.e. "I accept that there are not any gods."

One of the key things to remember, here, is that knowledge is a subset of belief. One cannot have knowledge of something, "I know this thing to be true," if one does not also believe that something, "I believe this thing to be true."