Gengar takes the most damage from Dark, Psychic and Ghost type attacks. It is also weak to Ground type attacks unless it uses its special ability Levitate, which renders Ground type attacks against it useless.
Gengar is a Poison-Ghost type of Pokemon. Haunter (#93) evolves into Gengar when traded. Gengar is also the final form of Gastly. In Pokemon Generation 4, in versions Diamond, Pearl and Platinum, it can be obtained by catching it in the Old Chateau.
Gengar has a strong resistance to Grass, Poison and Bug type attacks. It takes no damage from Normal and Fighting type attacks.