Yup. Earlier this year they made some changes to Mega Evolution. I have some posts about it, but they're kinda older, if you'd like to read more about it. A brief explanation: At Mega Level 3 they give +2 Candy, +100 XP per catch and an increased chance for XL Candy for Pokémon of the same-type as your currently active Mega. So an active Mega Gyarados provides those bonuses to Water-, and Dark-type Pokémon 😄