In White forest. If it took you many days to find white forest for the first time, there will be many trainers and wild Pokemon to find. If not, then you need to invite them from someone else's black city to Your White Forest using the entralink. This should help: http://bulbapedia.bulbagarden.net/wiki/White_Forest
Go to lavendar town and then enter Pokemon tower in there you can find a gastly.
No, Gastly cannot learn to fly.
Gastly is found in Bellsprout Tower.
Gastly evolves at level 25.
you find gastly in the Burned Tower and the Sprout Tower
Gastly is unobtainable in Pokemon Ruby. You need to trade one from Pokemon Firered or Leafgreen.
Go to lavendar town and then enter Pokemon tower in there you can find a gastly.
No, Gastly cannot learn to fly.
Gastly is found in Bellsprout Tower.
Gastly evolves to Haunter at level 25.
Gastly evolves at Level 25.
Yes you can get gastly I dont no about duskull
Gastly and Misdreavus on pearl. Gastly and Murkrow on Diamond.
Gastly evolves at level 25.
Gastly Lvl.25>Haunter Trade>Gengar
Gastly evolves at level 20 into Haunter
you find gastly in the Burned Tower and the Sprout Tower