A gamete is, by definition, a single cell.
In animals a male gamete is called a spermatozoon (plural spermatozoa), or simply sperm or sperm cell. A female gamete is an ovum (pl ova), or unfertilized egg. (A fertilized egg is a zygote.)
In lower plants, such as mosses and ferns, the male gamete may be called a sperm or an antherozoid.
In flowering plants, the male gamete is a nucleus within the pollen grain.