How do you get a gold Magikarp in Pokemon Gold?

You can't get a Gold Magikarp in Pokémon Gold. There are no Shiny Pokémon in Pokémon Gold, Silver, or Crystal.

Shiny Pokémon were introduced in Generation III (Pokémon Ruby, Sapphire, and Emerald).

So, you won't find a shiny Magikarp in your copy of Pokémon Gold.