I've heard that distinction as well, but it always struck me as coming from a religious position and working backwards, as if there is something inherently special about belief in a god or gods separate from belief or disbelief in other things that lack evidence.

I don't have to explain that I'm gnostic in my disbelief of vampires even though if a vampire was biting on my neck I'd believe in them. If I saw a sleigh pulled by reindeer flying through the sky, I'd believe in Santa, but absent any evidence and lots of reasons to believe Santa is impossible as an all-knowing, seemingly time-stopping magical being, I don't think we need a qualifier like "gnostic" or "agnostic" when discussing disbelief in Santa, because it is "impossible to know."

Gnostic and Agnostic seems like gotcha terminology for religious folk that capitalize on the more scientific view that if there is proof/evidence something exists, I will believe in it, but until then I will use reason to believe it does not to suggest there is a class of atheists that seems open to the idea of religion and another that doesn't. In reality, if you're starting from the atheist side, it's more:

"I am certain gods do not exist in the same way I am certain vampires and Santa Claus don't exist, in that unless and until reliable evidence is available to suggest they do there is no reason to believe in them. But as with any of my beliefs, if reliable evidence or proof is offered I'm willing to reconsider my position."

The gnostic/agnostic atheist/theist distinction is quite useful. Also the words are fun to pronounce/spell so that's a bonus