Gnosis is a greek word that means knowledge. Theism is a belief in god and Atheism is a lack of belief in god. So there are four categories considering belief and knowledge of a god.
1.) Gnostic Theist is someone who believes in a god and is 100% certain they know it's true
2.) Gnostic Atheist is someone who doesn't believe in god and 100% certain there is no god.
3.) Agnostic Theist is someone who believes in a god but is not sure there is a god.
4.) Agnostic Atheist someone who doesn't believe in a god, but doesn't know if there is one. Most of us fit into category 4. I think it's a misnomer to seperate the two categories as full answers to the question do you believe in god. If you ask me if I believe in god I would say, "No, I don't believe in god, I am an atheist." If you were to ask me if there is a god I would say, "I have no earthly idea, I am an agnostic.

I doubt that you will ever get concensus on their meanings as there appear to be very different usages over time and distance.

I see no relationship between free thinker and gods. Free thinker for me is about being open to new ideas and being willing to consider and challenge the validity of social constructs.

A humanist is someone who considers how best to promote equality of all people in their best interest in the short and longer term.

Atheist does not accept the claims by theists that a god/s exists. It addresses belief. I see no evidence that god/s exists - the same that I see no evidence for unicorns, bigfoot or the loch ness monster and so do not believe in them either. When I am presented with credible, independently verifiable evidence, then I will believe they exist. Not before.

Agnosticism addresses knowledge. Do I know that in any universe at any time that a god/s cannot exist? No. I know that it is not possible to prove a negative - the null hypothesis - and that it could be possible that somewhere in the universe there could be something that may fit a description of a god. But I will not believe claims that there is without evidence.

Antitheist is the claim that god/s do not and cannot exist.

Dictionary definitions generally describe common usage and differ depending on where and when they were published as language is fluid and changes with time and geography.

It's in the "Questions" section you answer for your own Profile. I've copied that section from my own - rather than provide a link - since you can find it on your own within your own profile.

Which term(s) do you identify with?

While no definition is perfect, here are some:

Agnostic - sees no evidence for God(s)

Atheist - has no belief in any God(s)

Anti-theist - actively opposes religion and belief in God(s)

Humanist - rejects supernaturalism and focuses on human activities

Secularist - advocates separation of the state from religious institutions

Skeptic - questions religious authority and beliefs

Freethinker - believes truth comes from logic, reason, and empiricism, rather than religion

Spiritual - believes in a supernatural entity

Believer - believes that specific God(s) exists

Religious - believes that specific God(s) require worship