Final answer:
The gametophyte is the mature gamete-producing structure of a moss gametophyte.
Step-by-step explanation:
A gametophyte is one of the two alternating phases in the life cycle of plants and algae that undergo alternation of generations. The alternation of generations involves the alternating occurrence of two distinct multicellular phases: the haploid gametophyte and the diploid sporophyte. The gametophyte is the haploid phase, producing gametes (sex cells), and it gives rise to the sporophyte through sexual reproduction.
In mosses, the gametophyte is the mature gamete-producing structure. It is the dominant and most visible stage in the life cycle of mosses. The gametophyte produces a form called the gametangium, which is responsible for producing gametes.