I am not a fan of gravel for chick runs. No personal experience with it, but rocks get hot, can have sharp edges (increasing the chance of foot problems) and IF they are dumb enough to try and eat one of the smaller ones and it's too big can cause big problems in their digestion. Yes, chickens need gravelly stuff in their crop to "chew" their food, but not actual gravel, small pebbles.
If I had to do it all over again, (and I don't, thank god) I have a load of sand delivered and put THAT in my run, instead of the compost I put there....admittedly the compost is turning into wonderful rich black soil, but it compacts so much they can't really dig in it and they definately can't dust bath in it. I did put sand from the coop in a hole that had developed as the compost compacted and they love to dust bath in that hole....but being a hole, it is still wet this time of year. Some people put play sand in an old kiddy pool for their birds to dust bathe in. That works too. Dust bathing is how chickens attempt to keep bugs out of their feathers.
My run is 100% surrounded by hardware cloth to make it predator proof, cuz I am very lazy and didn't want to have to shut the pop door every night to keep creatures out of the coop, so I had to put something IN my run. At other people's houses I have seen the run just the dirt that is on the ground outside the coop. Their chickens do fine with that. (My chickens are admittedly spoiled, but they still taste good when they piss me off. I have a large freezer. LOL)