Here are level 120 Mega Gyarados' stats since no one has posted them yet. First move I always use is Dragon Dance so you might as well multiple Attack and Speed by
Who is stronger between Mega Gyarados and Mega Garchomp? - Quora
Mega Gyarados • Generation VIII Pokédex • Marriland.com
Gyarados (Pokémon GO) - Best Movesets, Counters, Evolutions and CP
Gyarados | Wiki | Pokémon Amino
Pokemon: 10 Things You Didn't Know About Gyarados
Magikarp Pokédex: stats, moves, evolution & locations | Pokémon Database
What if Mega Gyarados was Water/Dragon instead of Water/Dark? How competitive would it be? - Pokemon Showdown - Quora
Tentacruel, Gastrodon, Milotic, Gyarados, or Azumarill for Diamond? - PokéBase Pokémon Answers
Gyarados Pokédex: stats, moves, evolution & locations | Pokémon Database