Plastic and gravel wouldn’t be a good idea. Your shed will sink/move/sit in water/rot. A shed base really needs to be solid so as a minimum, Id put down a bed of paving slabs first and then the shed on top.
My shed is on a concrete base with re-bar in it so nearby tree roots don’t crack the concrete base. The shed itself then sits on thick wooden beams on the bottom so it is raised off the concrete bed even further. My concrete base sits two inches higher than the surrounding ground so, again, it doesn’t sit in water.