To get Gastly you have to go to sprout tower at night.
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I have herd ghastly is in the sprout tower
you cant u need to catch a ghastly in the sprout tower and evolve it. it is in there at night only.
go to maps 23 or 24
Bellsprout tower at night time.
You can find it in Pokemon Platinum, Pokemon Crystal, Pokemon HeartGold, and Pokemon SoulSilver.