Short answers, pulled from the story.
The gametophyte is the haploid, sexual stage of the life cycle that has been overshadowed by the towering trees and lush ferns we see around us. It is the active engine of sexual reproduction that develops specialized organs to produce gametes which will eventually fuse to create a new diploid organism.
The gametophyte and the sporophyte of Ulva lactuca are externally indistinguishable in a phenomenon known as isomorphism where both stages look identical to the naked eye. The gametes are isogamous meaning they are all of one size shape and general morphology making it impossible to tell which is male and which is female without microscopic examination.
The moss gametophyte is the star of the show and a long-lived nutritionally independent organism that supports its own reproduction. It is the most visible stage of the life cycle and produces sex organs called gametangia including eggs in archegonia and sperm in antheridia.
The gametophytes of certain ferns such as Ophioglossaceae and Psilotaceae live underground and subsist by forming mycotrophic relationships with fungi. They derive nutrients from a symbiotic partnership rather than photosynthesis which is a radical departure from the typical photosynthetic free-living autotrophic organism called a prothallus.
The female gametophyte in angiosperms is also known as the embryo sac and is reduced to a mere seven cells and eight nuclei. It develops from a diploid megaspore that undergoes meiosis to produce four haploid daughter cells and then divides by mitosis to form the mature embryo sac.
In heterosporous plants the egg-producing gametophyte is known as a megagametophyte because it is typically larger and the sperm-producing gametophyte is known as a microgametophyte. The microgametophyte travels to the vicinity of the egg cell while the megagametophyte remains within the ovule providing a protected environment for the developing embryo.