Still, I see a huge gulf between the believer mistaking gnosis for faith and the agnostic's (arguably) incorrect conclusion that this or that thing can not be known.
To me it would be an unbeliever's mistaking gnosis from faith. To me gnosis means direct knowledge and that suggests direct personal observation. Those who have direct personal observation of deity by some mystical experience don't think they have any need for faith on which to base their belief. This suggests that taking it on faith is what is done by those who believe without having had direct personal observation of deity. Careful inspection of the issue shows the problem of objective repeatable observations versus subjective internal observations. Some unbelievers suggest that those who have had direct personal observation hallucinated or lied.
One of the agnostic stance is that existence can't be proven. If left at that step any of us who believe but understand we could be in error could call ourselves agnostics. It's usually extended to withholding judgment until there is proof or refusing to participate until there is proof.
Another meaning of agnostic is lacking direct personal knowledge. Again if left at that step those of us without direct personal observation of deity could call ourselves agnostic until they do experience deity. It's also not the only step taken in most cases. Some do some don't have direct experiences and no one has ever figured out how to cross that bridge. it just happens or doesn't.
Anyways, the fact that it is not yet proven is not good evidence that it never will be proven. Every scientific discovery made today is something not proven for the entire previous existence of humanity. there's more to it than that as there are statistical patterns and such, but new discoveries do keep on happening.