On what Nya was saying:
I would agree - except we're in a fantasy world, so the laws of science and physics don't necessarily apply. Heck, we're talking about using your very sould to make magic, so I have to disagree and say that logic doesn't really come into it so much. Meh, I could be wrong, though.
In my opinion, also under Earth should be a sort of sub-thingy like Nature. Not to create it, of course - like Tarot said, Gnosis doesn't give life. Just to manipulate nature, like making vines that are already present wrap around your target instead of growing them out of the ground really fast. Or making trees move. But the nature has to already be there, because Gnosis can't be used to give life and make stuff grow. I'm unsure if wood or metal would fall under Earth as an element. Meh, I don't know...
Also, it's my opinion (I have far too many opinions for one person) that Fire + Air should also = will o' the wisp. Imagine the fun possibilities - leading poor PCs astray then attacking them in the dark of night. Ahem. Just saying.