According to the search results, Gengar is weak against Dark, Ground, Ghost, and Psychic-type moves. Therefore, the following are some Pokémon that are strong against Gengar:

• Alakazam
• Mewtwo
• Espeon
• Tyranitar
• Metagross
• Latios
• Groudon
• Shadow Ball Mewtwo
• Primal Groudon
• Mega Gengar
• Mega Alakazam
• Mega Tyranitar
• Shadow Mewtwo

It is also recommended to use a mixture of strong ground and psychic-type Pokémon when fighting Gengar.