How do you catch gyrados or Magikarp in black white?

You can't catch Gyrados or Magikarp in Pokemon Black and White! Those Pokemon were introduced in the first generation (Red, Blue, Yellow) and don't appear in Black and White.

However, there are other water-type Pokemon you can catch in Black and White, like:

* Tentacool and Tentacruel: Found in the sea, especially in the Driftveil Drawbridge area.

* Shellder and Cloyster: Found in the sea and in some caves, like the Undella Bay.

* Carvanha and Sharpedo: Found in the sea, especially in the Nacrene City area.

* Panpour, Pansage, and Pansear: Found in the forests and grass, especially in the Nacrene City area.

* Basculin: Found in the sea, with different forms depending on the location.

Keep exploring the Unova region, and you'll surely find lots of exciting water-type Pokemon!