Agnostic is not another word for being on the fence. Agnosticism is the philosophical position that the existence of any sort of metaphysical entity or force cannot be proved nor disproved. It is a qualifier. You can be an agnostic atheist, holding the position that if metaphysical claims cannot be proved then there is no reason to preoccupy oneself with a belief in them, or you can be an agnostic theist, having faith in the existence of some manner of almighty creator but being fully aware that there is no amount of conclusive proof that can offered to justify such beliefs.
Gnosticism, in contrast, is the claim to know the existing status of any metaphysical entity or force. Gnosticism appears to be more common among the religious. Many claim to know, without any doubt, that their concept of god exists. They will tout their book as definitive proof. Atheists tend to be less comfortable with such confidence. There are atheists, however, who will make such claims. They may even have decent arguments, but they take the conclusions further than they should (eg: the probability argument which demonstrates that with the number of deities that may lay claim to the exclusive title of creator of all existence, the probability of any particular deity existing is near zero). These may frequently be the people who can be identified by the sub-group of atheists referred to as anti-theists. Anti-theists do not only disbelieve in metaphysical entities or forces, but are aggressively opposed to the notion that such belief should be allowed to exist.
Personally, I find the gnostic position potentially hazardous regardless of whether it is theistic or atheistic in nature.