Although stems of portobello mushrooms are edible, they are commonly removed before cooking as they are unappealing and have a tough and fibrous texture. Gently pull the stalk, and it should come off quickly. Gills refers to the fine dark brown strips under the mushroom cap. It is edible too, but some people remove them before cooking as they will release a murky brown liquid that might affect the overall aesthetic and color of the final dish. However, I choose to leave them on as they provide some additional flavor and texture.