Using definitions that are currently in Wikipedia:
Gnosticism (from gnostikos, "learned", from Ancient Greek: γνῶσις gnōsis, knowledge; Arabic: الغنوصية al-ġnūṣīh) is the belief that the material world created by the Demiurge should be shunned and the spiritual world should be embraced (God's world)
Gnosticism states that spiritual world is more important than material world, and that we should embrace the spiritual world and pay less attention to material world.
However the "spiritual world" is defined is another matter, but it does take a stance that there IS a division between "spiritual" world and "material" world, and specifically that the material world was created by an entity from spritual world.
Since Gnosticism has more than one claim they make, it's hard to nail down what its opposite - Agnosticism should be.
One possibility is that Agnosticism would, in opposition of Gnosticism, state that we should NOT embrace "spiritual world" at the cost of losing focus on the "material world".
Or, it could be a stance that material world was NOT created by spiritual world, but this alone could have several subtypes (such as maybe the spiritual world was created by material world and not the other way round).
Or, it could be a claim that there IS no division between "spiritual" and "material" world.
However, none of these seem to clearly coincide with the "popular" definition of Agnosticism, which is typically the view that there can be no certainty about anything spiritual. However, this popular view inherently acknowledges the
possibility of the existence of spiritual world.
Gnosticism-Agnosticism axis is not a simple "yes - maybe - no" axis of position, but rather something that contains both a view on what the relationship of "spiritual" and "material" world IS, and also a view on what our relationship to spiritual world should be.
Theism and Atheism are easier to define; Theism is belief in at least one deity, while Atheism is defined as no belief in any deity.
Even so, though, Atheism can be divided into Weak and Strong Atheism*, which also can have sub-divisions, but the gist of it is that Weak Atheism is simple lack of belief, while Strong Atheism takes a stance that no deities CAN exist; that it is impossible by definition. Weak Atheism doesn't take any particular stance on whether it's possible for divine entities to exist - there's just no belief in any of them.
This is typically an example of materialism or physicalism, which are basically philosophical views that claim there is no division between "material" and "spiritual", or "mundane" and "divine", as the theistic point of view would call them. In a materialistic point of view, anything that exists is by definition part of the "natural" or "material" world; therefore even if an entity with "god-like" properties would be proven to exist, it would still - by definition - be a natural entity and thus, there would be nothing "divine" in it.
Interestingly, there is another end of theistic spectrum which also denies the division between material and spiritual world. Pantheistic world view, however, takes an opposite stance than Materialist/Physicalist world view, and states that instead of everything being material, everything is divine!
However, the Pantheism - Theism - Weak Atheism - Strong Atheism axis has only one correlation with Gnosticism - Agnosticism axis:
Pantheists and Strong Atheists can be neither Gnostics or Agnostics. This is because both Gnostic and Agnostic positions acknowledge the POSSIBILITY that there is both material and spiritual world, while Pantheist view takes the stance that everything is Divine (spiritual) and Strong Atheist view takes the stance that everything is material (or mundane); therefore making it impossible to be either certain that material and spiritual world exist (gnosticism) or uncertain about it (agnosticism).
Interestingly, as far as everyday life and relationship to the world and people around you goes, Pantheism and Strong Atheism are remarkably similar in many cases - probably because both of these world views remove any basis for treating other people differently based on their religion. That is not to say there can't be other reasons for discrimination...
Theists can be both Gnostic or Agnostic in their views, particularly regarding their everyday behaviour and their personal relationship between the Divine or Spiritual world they believe in.
Some Theists are Gnostic in the sense that they concentrate on the spiritual world, with less importance on the material world. In the same sense, some Theists are agnostics in that they don't think it makes sense to ignore the material world in favour of spiritual. Different religions have different stances on this, too. Christianity as a whole emphasizes the importance of Spiritual world, but most sects still think it's important to live your life in the material world as well as you can. Depending on the particular sect of Christianity, some think that your actions in material world affect your fate in the spiritual world, while some think your actions are irrelevant and only your belief and faith and way of thinking matter, regarding the salvation of your soul.
Judaism, on the other hand, is much more concentrated on the material world. They do not put the same importance on "afterlife" as most Christian sects do; instead their concept of "afterlife" is more about the memory of your actions in the material world. If you lived your life good, you leave a good memory of yourself... but I don't think they even have a concept of personal afterlife the same way Christianity does.
Islam does, of course, have a concept of afterlife. Their view on the relationship between life and afterlife (or material and spiritual world) is curiously schizophrenic, though. Not only do they put an inordinate amount of rules and restrictions on how a good muslim is supposed to live, many of the muslim sects seem to emphasize the importance of spiritual world (afterlife) to the material world (life), which is definitely a Gnostic point of view and one of the most important suicide bomber recruiting arguments used by terrorist organizations.
So there's some examples of both Gnostic and Agnostic world religions.
Hinduism, I believe, is another example of very much Agnostic Theistic religion - there's not so much a difference between "divine" and "mundane" worlds, but there definitely are deities in Hinduism.
Gnostic Atheists are much less common. However, Buddhist philosophies (at least some of them) commonly use spiritual symbolism, and that would make it a Gnostic Weak Atheistic world view. However, on the other hand some directions of Buddhism are closer to Pantheism than anything else, which leaves them indifferent about Gnosticism or Agnosticism.
In a way, I suppose some of the "new-age" cults/sects/religions could also be classified as Gnostic Weak Atheism, with focus on "spiritual world" but not particularly Theistic. At the same time some of them, of course, are very much Theistic, so there you go.
I myself would classify myself as Physicalist type Strong Atheist, and neither Gnostic or Agnostic, because by my definitions there is no division between Material and Spiritual that I could be either certain (Gnosticism) or uncertain about (Agnosticism).
*I'm just calling them Weak and Strong Atheism because that's the most convenient way for me to label them. No offense meant for anyone who recognizes themselves as "Weak" Atheist as defined here.