Agnosticism is sometimes even more misunderstood than Atheism, so let's get some definitions out of the way:
Atheism: the rejection of belief in the existence of a deity
Theism: the belief in the existence of a deity
Agnosticism: a lack of knowledge
Gnosticism: a possession of knowledge
Atheism deals with belief (or rather, disbelief), while agnosticism deals with knowledge. One common misconception about it is that agnosticism is some kind of middle ground, where you're undecided if you believe or not. If you are that kind of person, you might be surprised to find out that you are an atheist. If you do not belief in god, you are an atheist. There is no middle ground, and you can't "sort of" believe in god. Agnosticism is a claim of knowledge, it means that you don't know, and it is separate from atheism, but can be used to describe it. I am an agnostic atheist because I while I do not believe in god, I do not then claim that god does not exist (there is a subtle difference). Most atheists are agnostic, because to be a gnostic atheist (god does not exist) would be dogmatic and would require evidence to prove the claim. On the other hand one can be an agnostic theist where you believe in god however you do not claim that god exists. This is rare because most theists actually claim god exists and therefore are gnostic theists. Agnostic theists believe that god exists but don't actually claim that one exists.
So, a summary:
atheism: rejection of theism
theism: belief in god
agnosticism: lack of knowledge
gnosticism: possession of knowledge
gnostic atheism: "God does not exist"
- Requires evidence because a claim is being made.
agnostic atheism: "I do not believe god exists, but do not know for certain"
- Does not require evidence because it does not make a claim, but rather is an antithesis to a claim being made. The rationale being that if a claim about the existence of a deity is made, and you reject that claim (usually due to lack of evidence, but there are some with personal or emotional vendettas), you are in this category. You are not then turning around and claiming god does not exist, you are just saying that there is no evidence for a god's existence.
agnostic theism: "I believe in god, but do not know for certain"
- Requires evidence because it makes a claim, even though it is an agnostic claim
gnostic theism: "God exists"
- Requires evidence because it makes a claim