No, that is merely the way that you choose to use the words.
(a)theism and (a)gnosticism are two entirely seperate dichotomies. (A)
theism refers to whether or not one has faith in a theistic deity. (A)
gnosticism refers to how certain you are in your belief - about anything, not necessarily theism, although it is often used as such (see the first listing in the link). As an example, I would describe myself as a 'free-will agnostic' - I think that free will exists because I experience it, but I accept that this may be an illusion, so I do not 'know' that free will exists. Another example - some programming languages are described as being
'device-agnostic', because they do not need to know what type of device they are running on in order to work.
As such, there are four possible combinations:
Gnostic theist - one who believes in a God, and claims that their belief is backed up sufficiently to be termed 'knowledge' - eg they have personal experience which they consider is beyond question.
Agnostic theist - one who believes in a God, but admits they may be wrong in their belief. There are several examples of this; I believe that
Unitarian Universalism would probably fall into such a catagory (although I don't know enough to be sure - ironically enough I am agnostic about that claim)
Agnostic atheist - one who does not believe in a God, but admits that they may be wrong. This describes the majority of 'atheists', although there are those (often those who are theistic themselves, oddly enough) who would have you believe otherwise.
Gnostic atheist - one who does not believe in a God, and claims their absence of belief is backed up sufficientlyl enough to be called 'knowledge', which could also be described as a 'belief in the absence of God' by the cynical. There are not many of these (simply because ultimately, absence of evidence is not the same as evidence of absence) but they do exist. It is also much easier to be a gnostic atheist when 'theism' is reduced to a specific form of God - if that reduction causes a logical inconsistency. For example, an all-loving, all-powerful God does not logically fit with the idea of people being irredeemably doomed to suffer in Hell if they make the wrong choices in life, so I might describe myself as having a more active belief that such a situation does not exist.
You don't like these definitions? Don't use them. But don't try and deny they don't exist.