Gastly Eggs hatch in 20 Egg cycles.
Generation II 5120 steps
Generation III 5376 steps
Generation IV 5355 steps
Generation V/VI 5140 steps
Generation VII 5120 steps
Generation VIII/IX 2560 steps
List of Pokémon by breeding attributes (Bulbapedia) Decreasing Egg cycles (Bulbapedia)
21 Egg cycles (minimum 5,355 steps). Source: http://pokemondb.net/pokedex/gastly For more on breeding: http://pokemondb.net/mechanics/breeding
