"Gnost" comes from the Ancient Greek word gnostikos for learned or knowledge. So the term gnostic means to know something, and its counterpart is agnostic, meaning lacking knowledge.
I admit when I was first having doubts about religion and god, I threw the term agnostic around to describe my stance because someone once told me they were one and their doubts matched mine, but that wasn't complete. The more I researched the more I came to realize agnostic has nothing to do with religion or gods. I can be agnostic about a lot of things, such as French ballet and oil rigs. When it comes to gods, I assert everyone is agnostic, because how can you absolutely know if a god exists? They can pretend they know, or make some immature irrational statement from personal revelation, but in the end it's hokum.
Many believe agnostic means you reserve the right to believe in a god should we find proof of one. But clearly that's wrong. Atheists have the same right.
A theist is someone who believes in a god and an atheist is someone who doesn't believe in a god. That doesn't mean an atheist couldn't believe if evidence proved a god existed, in fact it means exactly the opposite. I would gladly admit there was a god if one presented itself. Often, believers list their beliefs, superstitions, holy books and ignorance as proof, and when atheists carefully point out none of these things are evidence, they ask, "What evidence could a god provide that would make you a believer?" I like to say the deity/creator/god would know what level of evidence it would require to make everyone believe, so why answer the question?
There are some atheists who will claim there is no god, but that only hurts their stance, because then they would need to prove their claim, which they can't. So to say a gnostic atheist exists is just as unlikely as a gnostic theist. We'll never know for sure one way or the other unless a god did what was required to erase doubt. But real evidence mounts heavily on the side of no existence of gods.
Me? I'm an agnostic atheist, because the two terms aren't mutually exclusive and it's the most honest description of my worldview. I don't know if any god exists and I believe there isn't one based on the available evidence.