Gastly evolves into Haunter starting at level 25. Once Gastly reaches level 25, it will automatically evolve into Haunter unless it is holding an Everstone, which prevents evolution
. After evolving into Haunter, to evolve it further into Gengar, the player must trade Haunter to another trainer; this trade triggers the evolution into Gengar
. If you catch a Gastly that is already level 25 or above, you may need to level it up once more to trigger the evolution into Haunter
