Weak toFightingBugGrassElectricFairy ×1.6

Attacker	Type	Moveset	DPS
Regieleki	Electric	Lock OnThunder CageLegacy	26.3
Lucario	FightingSteel	Force PalmAura SphereLegacy	23.6
Pheromosa	BugFighting	Bug BiteFocus Blast	22.9
Kartana	GrassSteel	Fury CutterLeaf Blade	22.7
Zeraora	Electric	Volt SwitchPlasma Fists	22.6
Zekrom	DragonElectric	Charge BeamFusion BoltLegacy	22.1
Xurkitree	Electric	Thunder ShockDischarge	21.8
Terrakion	RockFighting	Double KickSacred SwordLegacy	21.5
Blaziken	FireFighting	CounterAura Sphere	21.3
Tapu Koko	ElectricFairy	Volt SwitchNatures MadnessLegacy	20.7
Breloom	GrassFighting	Force PalmGrass KnotLegacy	20.2
Enamorus	FairyFlying	Fairy WindDazzling Gleam	19.9

Rainy boosts the Electric attacks above. Fog and Rainy boost Mega Gyarados.

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

Mega Gyarados's moveset, stats and CP All counters Raid board

Questions people ask

What is Mega Gyarados weak to in Pokémon GO?

Mega Gyarados takes 1.6 times from Fighting, Bug, Grass, Electric and Fairy.

What's the best counter to Mega Gyarados?

Regieleki with Lock On and Thunder Cage, worth 26.3 damage per second against Mega Gyarados's own defense. The full ranking is above.