A Gender determines whether the Pokémon is male or female in the Pokémon video games. Some Pokémon have a greater chance of being one gender than another, such as Blastoise which has a 87.5% chance of being male, but only a 12.5% chance of being female. There is also moves that will only work when Pokémon are certain genders. There were no genders in the Generation I games.
There are some Pokémon that have no Gender, usually because they are made of lifeless materials like metal and clay. Some of there are Voltorb, Electrode, Beldum, Metang, Metagross, and Ditto. There are also no Legendary Pokémon with genders, except for Latias, Latios, Heatran, Cresselia ,Landorus,Thundurus and Tornadus.