This will be much better once I post a photo of my snowtires sitting beside my gravel tires...but that will come later. In the meantime:

Gravel Tires have a tread pattern that is more widely spaced then your winter tires. The tread blocks are deeper as well, and have a much stiffer block to them. The sidewalls are also significantly stronger, which keeps the tire from rolling over as you load them sideways while cornering. Basically the gravel tires will 'squirm' less and thus have better traction.

Running your snow tires is fine, I ran snow tires in the MLRC mini rallies and the first two seasons of rallycross. Your snow tires are more then fine for getting a taste for it and deciding if investing in gravel tires (likely used) is a wise choice. No point in buying them if you're only going to use them once.

Don't dismiss your street tires just yet! I ran on my Yokohama A520's one year because it was so dry and loose the wider tire helped the car to 'float' up on top of the surface better. My times on my A520's were faster then when I switched to the winter tires. Mind you, I didn't care about the rims I had them on.

-Dave