The Gold Magikarp is a shiny Magikarp. It evolves into a red Gyarados.
There is a 1 in 8912 chance that the Pokemon you encounter is shiny. This can be reduced to 1 in 2048 chance by using the Masuda method. This is basically when you breed a Magikarp with a foreign Ditto. The egg has a 1 in 2048 chance of being shiny.
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It's a level 100 Magikarp.
Beat the grunts
Level 21
Just be lucky. Keep fishing :P
Level20, As it always does in every other Pokemon Game.