Atheist and theist refer to whether someone believes in any kind of theism. Anti-theism and pro-theism would take it further in terms of whether you want to promote or reduce the amount of theism.
Agnostic and gnostic merely indicate a level of certainty in any belief. Its extremely rare that people are perfectly neutral between atheism and theism. They usually lean in one direction or another, so agnostics are either agnostic theists or agnostic atheist. They are usually the latter, as they are also often atheists trying to minimize the social costs of being a non-believer.
They are terms for different axes of belief.
Atheist and theist refer to whether someone believes in any kind of theism. Anti-theism and pro-theism would take it further in terms of whether you want to promote or reduce the amount of theism.
Agnostic and gnostic merely indicate a level of certainty in any belief. Its extremely rare that people are perfectly neutral between atheism and theism. They usually lean in one direction or another, so agnostics are either agnostic theists or agnostic atheist. They are usually the latter, as they are also often atheists trying to minimize the social costs of being a non-believer.