One of the commonly misunderstood positions on the proposition of the existence of a god is agnosticism. It is often incorrectly perceived that agnosticism is somewhere in between atheism and theism and that it amounts to undecided or is some less dogmatic version of atheism.
The fact is that there are no other possible positions with regards to belief in a god than theism or atheism. You either have belief or you lack belief. Being uncertain is not having belief. Furthermore, theism is a position of belief while gnosticism is a position of knowledge. So, what is the difference between belief and knowledge?
Belief: the psychological state in which an individual holds a proposition or premise to be true
Knowledge: justified true belief
Truth: a verified or indisputable fact, proposition, or principle; conformity with fact or reality
It is important to note that the definition of belief, above, does not require that a proposition is true. It is also not necessary that an individual has justification for maintaining a belief. Knowledge, on the other hand, requires both. The diagram below depicts the relationship between knowledge, belief, and truth.
Since belief and knowledge are not mutually exclusive, and since theism and gnosticism are positions of belief and knowledge, neither are theism and gnosticism mutually exclusive. The diagram below shows how these various positions on the proposition of the existence of a god may overlap.
This is why some atheists refer to themselves as agnostic atheists. This means that they lack belief in a god and have no knowledge proving or disproving the existence of a god. I would argue that with regards to the existence of a god it is not possible to be gnostic. The proposition is not provable or disprovable so neither a theist nor an atheist may be gnostic on the subject. Of course this does not stop someone from claiming that they know with certainty one way or the other. Certainty is not knowledge though. It is necessary to prove that their position has justification and can be universally acknowledged as truth.
This is only a brief overview of a few aspects of epistemology. I highly recommend reading more about this topic if you are still uncertain where you stand or if you disagree with the conclusions presented here.