That’s a good question. It’s a bit complicated.
People on Quora and some social media are starting to use the word gnostic.
I strongly disagree with this. This is because “gnostic” is already a well-defined word. See here: Gnosticism - Wikipedia, and here: Internet Encyclopedia of Philosophy and here: CATHOLIC ENCYCLOPEDIA: Gnosticism
The problem is that gnosticism influenced early Christianity, and competed against it. So you’re often talking about it and Christianity in the same context.
People say that it’s fine to have a word with two different meanings. That’s true, but it’s not something we should encourage, and it’s not something that’s common when they’re talked about in the same context. If you’re talking about climate change, and someone says they’re a skeptic, you know they’re skeptical about the prevailing model of anthropogenic climate change. You don’t think that they are necessarily skeptical about the correlation between HIV/AIDS, for example. The word isn’t a problem because it’s clear from the context.
But in the case of gnostic, if I say that the gospel of John is strongly gnostic, in this context you won’t know what I mean, will you?
Also, it’s a neologism to use the word in this way, whereas “gnostic” as a group of mystical religions has a very long pedigree.
So the opposite of “agnostic” is not “gnostic”.
Traditionally, “agnostic” is a relatively new word (because most English speakers weren’t allowed to question the existence of God till the last couple of hundred years anyway!)
Also, usually, agnostic is seen as the opposite of either theist or atheist. In this sense, “theist” and “atheist” are the same, in that they’re certain (either certain that God does exist, or certain that God does not exist), while an agnostic is uncertain.
Some people now talk about being an agnostic atheist (“I can’t be certain that God does not exist, but I chose to act as if he does not”) or agnostic theist (“I can’t be certain that God does exist, but I chose to act as if he does”). The opposite of those things would probably best be just “atheist” or just “theist”. But you could say something like “confident atheist” or “confident theist”.