you find gastly in the Burned Tower and the Sprout Tower
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just catch a gastly in the tower before the first gym and level him til he evolves
You can't catch one in Pokemon Emerald.
you can find him there and in bellsprout tower
To get Gastly you have to go to sprout tower at night.
You can find Haunter in SoulSilver in Route 8 at night. Additionally you can catch a Gastly at night in the Sprout Tower or the Bell Tower and then you can train it until Level 25 which is when it'll evolve into a Haunter.