Mega Evolution also affects its brain, leaving no other function except its destructive instinct to burn everything to cinders.
– Pokémon Sun Pokédex
Mega Gyarados is still not a dragon type, instead it’s Water/Dark. Its Pokedex entry, like a lot of Pokedex entries for mega Pokemon, makes you feel bad for it, since Mega Evolution seems to mess with its body =(
Stat-wise, it gains more Attack, more defenses, and 10 more Sp Atk upon mega evolution; its Speed is unchanged.
In the game, Lysander has a Mega Gyarados. In the anime, it was a shiny one. In the anime, Misty is also shown to have a Mega Gyarados.