The etymology fairly strongly suggests that a-theism is a lack of belief in theism, and a-gnosticism is a lack of belief in gnosticism.
Actually, the etymology of agnosticism is rejection of gnosis, which is rather broader than the gnostics. We know that because Huxley said so when he coined the word in 1869. That's also exactly the meaning RobinZ gave.
This is our monthly thread for collecting these little gems and pearls of wisdom, rationality-related quotes you've seen recently, or had stored in your quotesfile for ages, and which might be handy to link to in one of our discussions.