Levitate. Ground-type moves don't affect it.
In Silver, Gastly's have not yet picked up the ability to levitate plus Bite (and Dark moves) are now super effective. So basically use Dark Moves and with Gastly line being part Poison you can attack them with Ground Pokemon.
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 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 to Haunter at level 25.
Gastly evolves at level 25.
Gastly evolves at level 20 into Haunter
Gastly Lvl.25>Haunter Trade>Gengar