In Pokémon SoulSilver, Gastly will evolve once it hits Level 25.
Gastly evolves at level 25.
Gastly evolves into Haunter at Lv25.
in the lv 26
Level 26
Levels Pokemon evolve range from 7-55.
Level 30
level 30
You don't need to get a gastly to evolve because you can catch haunters in Pokemon tower
Well, im not sure about higher levels but you can catch a gastly at the burned tower in Ecruteak. hope i helped.
gastly
Pokemon that evolve with levels have certain levels, all of them are different. Some don't evolve by levels, and either have to be happy or use an evolutionary stone.
You can find a gastly in sprout tower at night in heart gold in soul silver i dont now.