Gyarados

Lead Gyarados
Item: Leftovers
Nature: Adamant
EV's: 216 HP, 16 Atk, 176 Def, 102 Speed
Moveset:
Dragon Dance
Taunt
Waterfall
Earthquake

Strategy Gyarados has many movesets this is one of my favourite ones. Intimdate stops leads such as Heracross and Machamp from harming your team. They will be made to switch in because of the attack drop from Gyarados' Intimidate. Taunt stops leads such as Bronzong which try to setup Stealth Rock or try to put you to sleep with Hypnosis. Both will be forced to switch out. Earthquake is preferred on this set to beat incoming Electric Pokémon. Waterfall is obviously there for the STAB (Same Type Attack Boost). Watch out for Pokémon such as Cresselia and Rotom since they dodge Earthquake and can have electric moves.

DD-Gyarados
Item: Leftovers
Nature: Adamant
EV's: 72 HP, 252 Atk, 184 Speed
Moveset:
Dragon Dance
Waterfall
Earthquake
Ice Fang

Strategy This Gyarados is all about sweeping. Switch in on a Pokémon that really cares about the attack drop from Intimidate. Then Dragon Dance on the switch. If you are taking only a little bit of damage from your opponents attacks then you could try Dragon Dancing again and again until you think its enough. Waterfall gives you the STAB. Earthquake handles the electric Pokémon and Ice Fang takes out the dragons and various other types.

Counters
Cresselia with Charge Beam is an obvious counter for Gyarados but is not guaranteed an OHKO. Watch out for Electivire when facing a Gyarados. They may predict an Electric move coming and switch into an Electivire. Basically Gyarados is not the easiest Pokémon to counter. Your best bet is to stop it before it sets up otherwise your in big trouble. Ask Bennet if you don't believe me .