You don't know what you believe, or you don't know the answer to the question "does God exist?"
Edit - to try and simplify, lets ask two questions:
1. I know with reasonable certainty whether God(s) exist(s) or not
(a) True
(b) False
Anyone answering (a) is gnostic, anyone answering (b) is agnostic. Notice that the question is a choice between having knowledge or lacking knowledge. If you claim to KNOW, then you are a gnostic. For all other states like "I'm pretty sure but not certain" then you are agnostic. There are no shades of grey in between the two states where you can be "partially gnostic".
2. I believe that God(s) do(es) exist
(a) True
(b) False
Anyone answering (a) is theist, anyone answering (b) is atheist. Once again, it's a choice between having a belief or lacking the belief. If you do believe then you are a theist, regardless of the certainty of your belief. For all other states like "I don't have a belief one way or another" then you are an atheist. Just as with gnosticism, you cannot be "partially theist".
Say a car pulled up in front of you. Very easily, you would KNOW whether the car is red or not, and you would believe the same thing as you know. It is a red car. Now say you've got a blindfold on and a car pulls up in front of you. You have no way of KNOWING whether or not the car is red or not, so you are agnostic. If you want to make a leap of faith and believe that the car is red then you are a "red-car-ist". If you lack the belief that the car is red, either because you believe it's blue or because you refuse to commit to any belief about the colour of the car or you think it is actually a bus, then you are an "a-red-car-ist". As in you are simply not a "red-car-ist". You have no positive belief that the car is red.
*Just a footnote, I'm using theism here as shorthand for a broad range of beliefs of the general nature "God(s) exist(s)", to cover pantheism, polytheism, deism, etc etc. I'd write out all the variants each time but it would turn into one hell of a long post....
Edit 2 - that chart I first posted is probably a bit misleading, here's a more accurate one:
Anyone on the blue line at the top is gnostic, anyone anywhere else is agnostic
Anyone on the red line at the right is theist, anyone anywhere else is atheist.
Anyone in that top right corner is a gnostic theist.
Anyone anywhere in the middle of the box for any reason is an agnostic atheist.