While I did already define their counterparts, I will try to explain what being a gnostic means without using the "opposite of agnostic" argument. Also, before I start, a small disclaimer - being a gnostic doesn't mean being religious!
~Core belief: To Know is not only possible but achievable or already achieved! From this possible pool of knowledge self awareness is removed, at least while I am writing about it simply because we logically determined that that is something that everyone can know and be aware of.
~What a gnostic should believe in: Faith is of less importance to him then to an agnostic.
Now you are thinking "WTF am I talking about?!" but think about it. Gnostic knows somethings is there. As knowledge is to be consciously aware of a fact with ultimate certainty.. gnostic doesn't need to believe in anything that he knows is true. Only thing that he can believe in is the interpretation of that knowledge. Otherwise, that ultimate knowledge is either ultimate and not dependent on faith or it's not ultimate and therefor not knowledge but an assumption.
~What's the issue then with gnostics? - aka Why do people think that agnosticism gives them some moral high ground?
Now this is where the fun actually starts! The issue many have with gnostics today can be represented not with what they believe in, but what they exclude as a possibility, apart from the idea their system is flawed.
This problem has nothing to do with To Know process, people! This problem is connected to bipolar process of thought trapped in a world of an idea that only one answer is correct.
In practice this means that people mostly look for an answer to something in a predetermined patter of option A or option B. And since there can be only one right answer that means that A is aB and B is aA (theists and atheist, gnostic and agnostic). So much are people trapped in this that today it's not possible to provide an alternative for then both sides will think you exclude their options and you will find your self under the attack from both A and B.
Agnostics exclude the possibility that anything is right, gnostics that nothing is right, atheists exclude the idea that there is a higher power, theists exclude the idea of an universe without a higher power - all fine and dandy.
All the problem arise when then want to exclude, cancel and delete others and this problem is derived from the bipolar way of thinking and the unilateral logic.
Think about it! Think what you have done and learn from it! Think what you will do and prepare for it!