In its debut appearance after evolving into a Magikarp, Gyarados was seen as an ill-tempered Pokémon with uncontrollable rage similar to James' Gyarados from the episode, Pokémon Shipwreck!. It was demonstrated when he was out of control, such as scaring Misty's other Water Pokémon beneath the pool, and like Ash's Charizard, he hits Misty several times with Hydro Pump, just like how Charizard hits Ash's face from its Flamethrower, and Misty did her best to tame it despite her fear of it since she was a child.
Due to its initially hostile behavior, Misty had to lock Gyarados in a cage, which upset it. When the Invincible Pokémon Brothers challenged Misty, they used underhanded tactics to try to win, and became enraged when Misty was hurt trying to protect it. Gyarados became happy when Misty was safe and finally obeys her when she needs it in battle as seen in A Togepi Mirage! where it was released by Misty to battle Hansen's Shedinja, and to surprise Ash from its appearance, not to mention having learned Flamethrower itself to break through Shedinja's Wonder Guard ability that rendered all attacks but its weaknesses ineffective on it.
In the Sun & Moon series, Gyarados maintains its ill-tempered behavior and shows its wild side when Misty mega evolved it into Mega Gyarados and it started chasing Ash's Pikachu beneath the waters to hunt down its prey.
When Misty first returned to Cerulean City, Gyarados was attacking the trainers who were looking after the Gym for Misty's sisters while they were away. Gyarados was in a such a rage it wouldn't return to its Poké Ball, forcing Misty to lock it in a cage for its safety and others, upsetting it more. During Misty's battle with the Invincible Pokémon Brothers over ownership of the gym, Gyarados watched as the 3v1 battle begun with each of the brother's possessing their own Tentacruel. Gyarados grew concerned for Misty's Corsola during its battle, but when Misty used her body as a shield to protect Gyarados from the Tentacruel's attack, it broke free of its cage and defeated the brothers and their Pokémon itself with a Hyper Beam and Hydro Pump.
In A Togepi Mirage!, Gyarados was released by Misty to battle against Colonel Hansen and his Shedinja to surprise Ash and the group as Togepi who had evolved into a Togetic protects five of the Togepi from Shedinja's Solar Beam using Safeguard. Gyarados quickly defeats Shedinja by its newly learned Flamethrower as Hansen's left mustache and brow are burned due to its effects and was arrested for treason by Sara's royal parents.
In the movie, Master of Mirage Pokémon, Gyarados was released by Misty along with the others to fend off the Mirage Pokémon created by Dr. Yung after revealing himself as the Mirage Master.
In the Sun & Moon series, Gyarados was later released by Misty to fight against Ash's Pikachu at the Cerulean City Gym. Misty reveals a Key Stone hidden behind her hairclip of her ponytail as she Mega Evolves her Gyarados into Mega Gyarados which is enough to deal Pikachu beneath the water using Crunch, but it was still hit from its Iron Tail. Misty had Gyarados use Hurricane in an attempt to trap Pikachu inside, but he managed to escape. Ash later had Pikachu use the Electric Z-Move Gigavolt Havoc, which Gyarados made no attempt to dodge. The attack dealt massive damage; unlike Mega Steelix who had endured the Fire Z-Move Inferno Overdrive from Turtonator, Mega Gyarados was unable to endure the Gigavolt Havoc and fainted. Misty returned Gyarados to its Poké Ball and thanked it for its effort.
Gyarados was later released by Misty during her and Brock's visit in Alola to battle Team Rocket's robot along with Ash and his classmates' Pokémon to rescue a school of Mantine which serves as their Poké Ride. Misty mega evolves her Gyarados and hits the robot with Hydro Pump as the final blow after everyone unleashes their respective Z-Moves.