Pokemon-go – What are the most effective evolutions regarding EP

When you evolve a Pokemon, the CP scales to the same percentage of your max at that level.

You can tell the approximate percentage by looking at the circle above a captured Pokemon. After evolution, the marker will always be in the same place.

For example, if you're evolving a Pokemon with a Max CP of 100 to a Pokemon with a Max CP of 300, your current CP will triple to keep the same ratio.

Yes. Gyarados is accessible in the wild.

As it does not have a rarity class¹ (note the PokemonClass on Mew or Mewtwo) assigned to it, it may spawn freely (albeit rarely) across the world.

However, due to their spawn chances, it may not be as much work to evolve a Magikarp -- unless you're exceedingly lucky and near one of the biomes in the game that supports Gyarados.

^{1The data sourced in the above dump came from packet and game reverse-engineering and has since been validated by many people, including myself. It is perhaps, as of now, the best source of information we have on what makes Pokemon tick.}