good call, my shop floor was slightly slopped to a center floor drain I designed in the slab so I didnt do the gap.Hold the plywood at least 1/2 inch off the concrete. Any water that gets in the bldg will leach up into any sitting on concrete.
What is the span between C channel ?
That will also dictate 1/2 or 3/4 plywood . If running 1/2 and the span is more than 2 feet I would back the joints with 2x4 to keep the ply from warping . The metal clips will let it warp in those good old Okie muggy months.