Gnosticism is not a position for or against belief in God it is a position on knowing.

People can be Agnostic Theists and Agnostic Atheists, but it is not an exclusive concept on its own.

Theism/Atheism is a position on the belief of gods existence

Gnosticism/Agnosticism is a position on knowing of gods existence (when in relation to god it is not mutually exclusive to god)

Most if not all Atheists are Agnostic Atheists, And most if not all Theists are Gnostic Theists.
People who claim Agnosticism, often don't actually know what Agnosticism is. They think it is that they are positively open to any and all religious doctrines. In reality it is the position that they don't know, and that there is no way to know without evidence. Sound familiar. . . . . . Atheism perhaps?

If someone tells you that they are Agnostic make a further inquiry by asking, "okay, and what is your position on the belief in god?" If they respond with, "I already told you I am an Agnostic" this means that they have no clue what it actually means.

Theism and Atheism are the positions that Either you believe or you don't, Gnosticism and Agnosticism are the positions that either you know or you don't. There is not fence sitting position for god.

Either you believe and you say you know, or you believe and you say you don't know. (the second is a false faith in religion by their own definition)

And

Either you do not believe and you know that there is no god, or you do not believe and you don't know if there is a god. (both are Atheistic, but one is logically honest and the other really isn't) absence of any evidence means you can't know for certain. So to contend that you don't believe in god and that you know for certain that all gods do not exist is an illogical assertion that cannot be proven. Although you can assert that logical contradictions are in inherently false.  

Agnostics who define themselves separate from Theism/Atheism often do so in a way to justify a political correctness for what ever personal reason they may have. However, just because they feel there is some possibility that Thor could exist if given evidence, it does not mean that they believe in Thor. Now since agnostics can't techincally be both agnostic and theists due to the need to give possibility to all deities in being Agnostic, and Theistic denominations are exclusive ideologies, logically all Agnostics are Atheists. This would also make all Theist's exclusively Gnostic.

Now before you rip my head off let me explain the logic behind Theists being exclusively Gnostic. Theists from my experience, contend that only their god has a valid belief system. Exclusively Agnostic Theist's would have to be open to all deities, So it would be ignorant to claim to be an Agnostic Theist. Just as it is logically dishonest to claim to be a Gnostic Atheist in regards to all gods. From this point on you further explore into the semantics of defining yourself as a Agnostic christian and a atheistic muslim Jew.  

To say that you are just an Agnostic on the question of belief in god is based completely on the ignorance of the definition of Agnosticism as well as its etymology.