They are two different things.
The scale goes from Atheist to Theist, where atheist is having no belief in God and theist is having a belief in God.
The other scale goes from Agnostic where the person does not claim to know for certain to Gnostic where the person does claim to know for certain.
I am an agnostic atheist, though I lean towards the gnostic side. I don't have a belief in God, and while I can't claim that I know for certain that there is no God, I suspect very much that there is no God.
On the other hand, a Christian who says, "I know for a fact that God exists," is a gnostic theist. They have a belief in God, and they are certain that their belief is correct.
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