Ultra-sun: It’s said that gas emanating from a graveyard was possessed by the grievances of the deceased and thus became a Pokémon.Moon: Although Gastly is barely visible, when it’s near, a faint sweet smell lingers.Alpha-sapphire: Gastly is largely composed of gaseous matter. When exposed to a strong wind, the gaseous body quickly dwindles away. Groups of this Pokémon cluster under the eaves of houses to escape the ravages of wind.
Prof. Lotus' Notes: Ghastly is a portmanteau of Gas and Ghastly. Some Pokedex entries state that Gastly is born from poisonus gas, and Gastly with it's evolution line are the only Poison Ghost types as of Gen 7.