On the way to the Safari Zone on Cliff Cave (after defeating all 16 gyms), go down the ladder in Cliff Cave, use surf, keep going left and you will find a big gap in between some rocks and go through them. You have to rock climb to reach it though.
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catch it at the burnt tower
burnded tower
you find gastly in the Burned Tower and the Sprout Tower
I have herd ghastly is in the sprout tower
you can find him there and in bellsprout tower