Ghastly evolves into Haunter at level 25. Then Haunter evolves by trading.
Gastly evolves at LV. 25
Gastly evolves at level 20 into Haunter
Gastly evolves to Haunter at level 25.
Gastly evolves at Level 25.
Gastly evolves at level 25.
Level 25
25
gastly evolve on level 25
Gastly evolves at level 22 then Haunter evolves by trading it.
Gastly evolves at Level 25.
Gastly evolves into Haunter at level 25, then trade it and it will evolve again into Gengar.
Gastly evolves to Haunter at level 25. Then, Haunter evolves into Gengar by being traded.