Gametes are pure because they are always haploid.
Gametes are sex cells. A gamete is either an egg, from a female, or a sperm from a male (appearances vary). When we say a gamete is haploid, we are actually saying that the gamete needs another gamete to form a whole cell. Think of haploid mean “half”, a gamete contains half the alleles needed for a cell.
There are two copies of each allele in an organism, one from the “mother” and one from the “father”. They are either dominant or recessive. Gametes do not possess homologous chromosomes, they instead possess only one chromosome of each type which is either dominant or recessive.
For example: a plant gamete may contain a “tall” allele of a gene responsible for the plant’s growth. If the gamete already contains the “tall” allele, it cannot contain another allele for another trait, therefore making it pure.