During fertilization, required on male gametes for their fusion with female gametes (PubMed:25111819). Probably initiates the fusion of gamete cell membranes by inserting part of its extracellular domain into the cell membrane of a female gamete (PubMed:20080406). Detected in sperm (at protein level). Detected in testis seminal ducts. HAP2/GCS1 family members mediate membrane fusion between gametes in a broad range of eukaryotes, ranging from algae and higher plants to protozoans and cnidaria, suggesting they are derived from an ancestral gamete fusogen. They function similar to viral fusogens, by inserting part of their extracellular domain into the lipid bilayer of an adjoining cell. Belongs to the HAP2/GCS1 family.