Gastly is found in Bellsprout Tower.
you find gastly in the Burned Tower and the Sprout Tower
To catch a shiny Gastly, there are three things you can try. First off, you can try catching a shiny Gastly in the wild. However, there is a 1 in 8192 chance of encountering a shiny Gastly and may take you hours to find one. Second, you can try breeding a shiny Gastly. There is a greater chance to find a shiny Gastly by breeding, but it still takes a few hours depending on how well you breed. Last, you can use your PokeRadar to chain Gastly. Depending on your luck and how well you chain, you can find a shiny Gastly within minutes or more. If your chain of Gastly reach up to 40 or more, your chance of finding a shiny patch is 1 in 200. A shiny patch will contain a shiny Gastly. You may even find several shiny Gastly from chaining. If you're really lucky, you can get a shiny Gastly from your first or second or even tenth chain.
Go to lavendar town and then enter Pokemon tower in there you can find a gastly.
You can find a gastly in sprout tower at night in heart gold in soul silver i dont now.
In the Pokemon tower or sprout tower.
evolve gastly.
sprout tower
You can find recruitable Gastly and its evolutions in Sky Stairway.
you can find him there and in bellsprout tower
Once you have the silph scope you can go inside Pokemon tower and randomly on the floors where its foggy gastly can appear. Also you can find gastly inside lost cave on five island.
haunter then gastly