Where to catch Gyarados

In Paldea (Scarlet & Violet)

• Casseroya Lake — Walk, Static Encounter
• East Paldean Sea — Walk
• North Paldean Sea — Walk
• South Paldean Sea — Walk
• West Paldean Sea — Walk

In Kitakami (Scarlet & Violet – The Teal Mask)

• Chilling Waterhead — Walk
• Fellhorn Gorge — Walk
• Kitakami Wilds — Walk
• Oni Mountain — Walk

In Hoenn (Ruby, Sapphire & Emerald)

• Sootopolis City — Super Rod, Walk

In Johto & Kanto (Gold, Silver & Crystal)

• Fuchsia City — Good Rod, Super Rod
• Lake of Rage — Good Rod, Static Encounter, Super Rod, Surf

In Kanto (Red, Blue & Yellow)

• Fuchsia City — Super Rod, Walk

Gyarados — full Pokédex entry