Rumblestiltskin
I'm fully aware of all that, which is why I said that gnosticism isn't between anything...cause gnosticism and agnosticism aren't just different concepts...they're complete opposites.
That isn't actually the case, though.
Agnosticism is not the opposite of gnosticism. It seems reasonable to think that would be the case, since "a" as a prefix often means the opposite of that thing. Like how atheism is the opposite of theism, or asexual is the opposite of sexual, or asocial is the opposite of social.
But agnosticism isn't the opposite of Gnosticism. In fact, you could be an agnostic Gnostic.
The capital G is useful for remembering this. Gnosticism (capital G) generally refers to religious beliefs involving a demiurge who created the world, and a greater God above that demiurge, and the importance of "gnosis" (esoteric knowledge of some sort). Used today, the term generally refers to Gnostic Christianity or similar beliefs.
There may be arguably be some other definition of "gnosticism" (little g) I'm not well-versed on, but Gnosticism (big G) is its own thing (of course there are many forms of Gnosticism).