In the early 50's, a book was written in Britain called "The Henyard," by Geoffrey Sykes.
In it he described a henyard covered in straw, doing the same job as your gravel. Essentially, it keeps the birds out of the mud and off the ground. 6-8" of straw is not too much and more wont hurt. You can think of it as an outdoor 'deep litter.'
But unlike gravel, which is rather sterile, the straw harbors an amazing array of life. Everything from seedlings to zoo-microbial organisms will thrive in this straw - providing the chickens with an array of micronutrients never imagined to exist in gravel beds. Once a month the straw can be raked over and after a year, the straw can be replaced with fresh.
The soiled straw, now broken down significantly, can then be sent to the fields and gardens to build the soil.
"The Henyard" is rather hard to find, I'm sorry to say, and may be considered rare. It was only published in limited numbers and merely good copies may run to $100.
I've seen pristine ones go for over $200 - when they can be found on these shores at all. I was very lucky to get my copy for around $25, and that was via a personal contact in Britain.
Sadly, Mr Sykes' theories came at a time when the poultry industry in Britain was abandoning fold (pasture) rearing of birds in favor of what we now call "industrial" methods.
His straw hen yard notion offered a cheap, natural way to increase the profits and health of Britain's flocks, at a time when post-war Britain needed it most.
Swept away in the rush to achieve big, clean agri-biz results, his detailed methods are all but forgotten today.
If I were to start an outdoor "chicken run," I'd make sure it had the following:
1. Predator proof fencing.
2. Windbreaking features
3. Sturdy draft free shelters/coops.
4. Excellent drainage and well percolated soil.
5. Straw as a flooring material - lots and lots of straw.