It is not possible to find a Ghastly in Pokémon Emerald. The only way for you to get one would be to trade.
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You can't catch one in Pokemon Emerald.
misdreavus you can get this from Pokemon leafgreen. Or shuppet/banette can learn it and even duskull/dusclops. gastly and all evolves can to
Go to lavendar town and then enter Pokemon tower in there you can find a gastly.
Gastly is found in Bellsprout Tower.
No, Gastly cannot learn to fly.