I would never put gravel in my chicken run. What will they do when shavings, poop and gravel are all mixed together? It won't ever compost... I imagine it would be unpleasant for chickens to scratch through, too. Oh well, to each his own I guess. They must have a plan. Just the roof with proper drainage should allow for a nice dry dirt run... with shavings if they want.They are going put about 6 inches of gravel down, to allow the rain time to soak into the ground without making everything muddy. On top of the 6 inches of gravel they will lay a 2 inch layer of pine shavings, so the chickens don't have to walk on gravel.
They are also installing a roof over their run, the roof will be installed with gutters, with the runoff from the roof being drained into a pond nearby through an irrigation system.