The gametophyte generation of plants produces haploid gametes. They grow from spores produced by the diploid sporophyte generation during the alternation of generations in plants, according to Encyclopaedia Britannica.
The gametophyte generation and the sporophyte generation alternate; the diploid sporophytes produce haploid spores, which grow into gametophytes. The gametes of different gametophytes join via sexual reproduction to form diploid seeds, which then grow into the sporophytes, explains Wikipedia.
In advanced plants, such as trees and grasses, the sporophyte is large and dominate; and, it lives a long time compared to the gametophyte. In bryophytes, such as mosses and other primitive plants, the gametophyte is the dominant generation.