Weak toGroundGhostPsychicDark ×1.6

Attacker	Type	Moveset	DPS
Mewtwo	Psychic	ConfusionPsystrikeLegacy	38.2
Blacephalon	FireGhost	AstonishShadow Ball	35.8
Alakazam	Psychic	ConfusionPsychicLegacy	32.6
Hoopa	PsychicGhost	ConfusionShadow Ball	32.5
Darkrai	Dark	SnarlShadow Ball	32.2
Chandelure	GhostFire	HexShadow Ball	32.0
Gengar	GhostPoison	LickShadow BallLegacy	31.9
Lunala	PsychicGhost	ConfusionShadow Ball	31.7
Groudon	Ground	Mud ShotPrecipice BladesLegacy	31.7
Azelf	Psychic	ConfusionFuturesight	31.6
Espeon	Psychic	ConfusionPsychic	31.5
Absol	Dark	SnarlBrutal SwingLegacy	31.3

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

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

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

Questions people ask

What is Gengar weak to in Pokémon GO?

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

What's the best counter to Gengar?

Mewtwo with Confusion and Psystrike, worth 38.2 damage per second against Gengar's own defense. The full ranking is above.