Level 26
Gastly evolves at level 25.
Gastly evolves into Haunter at Lv25.
in the lv 26
In Pokémon SoulSilver, Gastly will evolve once it hits Level 25.
You don't need to get a gastly to evolve because you can catch haunters in Pokemon tower
Level 18.
Level 30.
Level 30.
level 28
Level 25
Level 37.
at level 40 it should evolve into drapion