That "dimple" is a tapered center hole, used to steady the piston in a cam grinding machine while it was being manufactured. It serves no purpose once the engine is assembled. Those open holes in the gasket don't correspond to cooling passages in all blocks, so that gasket id made to fit more than one application. As long as the other head/block passages aren't blocked, just ignore the big holes. The gasket will seal properly without them.
Jerry
