Atheist/theist and agnostic/gnostic answer different questions.By denying the existance of a god you remove the agnostic. There is a difference between saying that there is no proof and that it is impossible to know and making an afirmation that they do not exist.
I provided the definition I don't know what more I can do if you are too dense to understand it.
Likewise if a person believes in a god then they are not agnostic.
Do you believe in any gods? No = Atheist, Yes = Theist
If you do not, do you think you can know there are definitely no gods? No = Agnostic atheist, Yes = Gnostic atheist
If you do, do you think the god(s) you believe in definitely exist? No = Agnostic theist, Yes = Gnostic theist