The upcoming Pokémon GO Community Day is your big chance to get Gastly. On Sunday, July 19, from 11:00 a.m. to 5:00 p.m. in your local time zone, this Gas Pokémon will appear more frequently in the wild, and there'll be a better chance those you encounter will be Shiny. Plus, if you evolve a Haunter (the evolved form of Gastly) into Gengar during the event—or up to two hours afterward—that Gengar will know Shadow Punch, an attack it can't normally learn.
Additional bonuses will be in effect, too, such as Incense activated during the event lasting for three hours.
Good luck searching for Gastly on Community Day, Trainers!