Weak toGroundGhostPsychicDark ×1.6

Attacker	Type	Moveset	DPS
Mewtwo	Psychic	ConfusionPsystrikeLegacy	76.4
Blacephalon	FireGhost	AstonishShadow Ball	71.6
Alakazam	Psychic	ConfusionPsychicLegacy	65.2
Hoopa	PsychicGhost	ConfusionShadow Ball	64.9
Darkrai	Dark	SnarlShadow Ball	64.5
Chandelure	GhostFire	HexShadow Ball	64.0
Gengar	GhostPoison	LickShadow BallLegacy	63.9
Lunala	PsychicGhost	ConfusionShadow Ball	63.5
Groudon	Ground	Mud ShotPrecipice BladesLegacy	63.3
Azelf	Psychic	ConfusionFuturesight	63.3
Espeon	Psychic	ConfusionPsychic	63.0
Absol	Dark	SnarlBrutal SwingLegacy	62.6

Windy boosts the Psychic attacks above. Fog and Overcast boost Gastly.

Level 40, 15 attack IV, costed against Gastly's own defense and typing, one per evolution family.

Gastly's moveset, CP and shiny All counters Raid board

Questions people ask

What is Gastly weak to in Pokémon GO?

Gastly takes 1.6 times from Ground, Ghost, Psychic and Dark.

What's the best counter to Gastly?

Mewtwo with Confusion and Psystrike, worth 76.4 damage per second against Gastly's own defense. The full ranking is above.