Letsgo Pikachu, where can I change to Gengar?

Gengar cannot be obtained directly by capture. There is no wild Gengar in the wild. You have to go to the Baokemeng Tower in Resource Town. After completing the story of Gaga Ga, you can meet Gastly or Haunter in it and communicate with your friends with Haunter. Then Haunter will evolve into Gengar.

Gengar acquisition method

Gengar can't catch it directly. Exchange with Haunter and communication with others can evolve into Gengar, but it will be sent to the opposite side, so it's best to find a friend you know to exchange, so you can exchange it back.