Joe Momma Do you mean the Gyarados is not showing in the Pokémon tab, or the Pokédex tab, or both? Are there possibly any filters active on the former?
If you did see the evolution notification, I don't see any technical way how the Pokémon could disappear. Maybe try restarting the app to be sure and using the search feature.
Note that Gyarados will appear in place of the now-evolved Magikarp, not as a new Pokémon at the top of the list.
