If you don't plan on keeping your mill in that one location, gravel will keep the mud down, but then you will have gravel mixed with mud and sawdust when you move your mill. When my friend brought his portable mill on wheels over, there was a lot of walking back and forth in the sunshine (a shed would have been awesome), a lot of sawdust, and a lot of boards to stack. Keeping the logs clean has a lot to do with the sawmill blade staying sharp and cutting efficiently. And not cutting into logs that have wire stuck in them.