Which process must occur before the gametophyte generation?

The process must occur before the gametophyte generation is Sporophyte. A sporophyte is the diploid generation of a plant or alga that has a double set of chromosomes. [ All land plants, and some algae, have life cycles in which a multicellular haploid gametophyte generation alternates with a multicellular diploid generation. In the Gymnosperms and flowering plants (Angiosperms), the sporophyte generation is the most prominent phase, comprising the familiar green plant with its roots, stem, leaves and cones or flowers. In the flowering plants, the gametophytes are very reduced in size, and are represented by the pollen and the embryo sac.

Air enters through the nose (and sometimes the mouth), moves through the nasal cavity, the  pharynx, the  larynx, enters the  trachea, moves through the  bronchi  andbronchioles  till the alveoli.