Gnosticism/Agnosticism are, for all practical purposes, useless terms.
Gnosticism is what you know.
Theism is what you believe.
The two words are splitting hairs, the difference is just the level of certainty. You don't wait for absolute knowledge to act. You act on your beliefs.
A-gnosticism is a lack of total knowledge
A-theism is a lack in a belief in a god.
The definition of atheism is a lack in a belief in a god, not a claim that gods don't exist. There are some atheists who believe that no god's exist, but that is something extra beyond atheism.
That would be agnosticism. Atheists claim there is definitely no god.