To evolve Gastly into Haunter just get it up to level 25.
gastly evolve on level 25
haunter then gastly
Gastly evolves at level 25.
Gastly evolves at LV. 25
You can't use a stone on gastly to make it evolve it evolves at level 22.
Gastly evolves into Haunter at level 25, then trade it and it will evolve again into Gengar.
Gastly evolves to Haunter at level 25.
Gastly evolves at Level 25.
Gastly evolves at level 25.
In Pokémon SoulSilver, Gastly will evolve once it hits Level 25.
Gastly evolves at level 20 into Haunter
Gastly evolves into Haunter at Lv25.