The day he started out on his Pokémon journey, he came off as an arrogant trainer, much like his game counterpart, Blue. His chosen starter Pokémon was Squirtle, which was actually the same Pokémon that Ash originally wanted to start with. He even had a group of cheerleaders who cheered him on in all his battles and would go so far as crying whenever he lost. He didn't walk on his journeys like most trainers did; instead he had his own sporty red convertible with a chauffeur. Gary loved when he got a chance to make fun of Ash by bragging about how far behind Ash was compared to him. He also gave Ash the pet-name of "Ashy-boy" (Japanese: "Satoshi-kun"), which occasionally seems to annoy him. His style of training at this point was to capture many Pokémon and to use the strongest, as a stark contrast to Ash who focused on developing a bond with his Pokémon.
Soon he would eat his words when he would challenge the Viridian City Gym Leader, Giovanni to a Pokémon battle for his final, but not necessary, badge in The Battle of the Badge. At first he seemed to be winning against Giovanni, knocking out most of his Pokémon. But then Giovanni sent out Mewtwo against the trainer and even let Gary use both his Arcanine and Nidoking against it. Mewtwo made quick work of the two Pokémon and Gary experienced his first (known) loss. However this still didn't change his arrogance. It would take a loss at the Pokémon League way later on to finally realize that he needed to change himself.
Gary eventually returned in A Tent Situation. Much unlike his former self, he congratulated Ash on his victory in the Orange League. Soon Ash challenged him to a battle with his Pikachu as his choice. Gary in turn used his Eevee. Gary managed to defeat Ash and then left on his journey through Johto.
In his few appearances during the Johto season, he seemed to have become the complete opposite of who he was. He became polite, intelligent, and respectful to others, including Ash. In Can't Beat the Heat! he battled Ash at the Silver Conference. Ash ended up winning the battle when his Charizard defeated Gary's Blastoise.
In Johto Photo Finish, Gary decided that he didn't want to be a Pokémon Trainer anymore, but instead wanted to be a Pokémon researcher like his grandfather.
In Putting the Air Back in Aerodactyl, Gary works on Sayda Island with Crystal. There, he revived an Aerodactyl from an Old Amber. At first the Aerodactyl was afraid of him and every other person on the island, but Gary soon gained its trust after he fed it some fruit that Aerodactyl used to eat.
Eventually, he moves to the Sinnoh region where he is currently conducting his research.
In Home Is Where The Start Is, Gary reunited with Ash and battled him with his Electivire and defeated Ash's Pikachu once more.
In Ill-Will Hunting! he was taking care of a group of Shieldon and protected them from the evil Pokémon Hunter J and her minions. He appeared again in Fighting Fear With Fear! where he helped Ash and Gligar by giving a Razor Fang to Ash which caused Gligar to evolve later. He was in the area after hearing rumors of Azelf. Upon meeting him, Dawn thought that he was a poet like his grandfather.
Gary later appeared in The Needs of the Three! and The Battle Finale of Legend!, working alongside Professor Rowan at Valor Lakefront where he attempted to protect Azelf from Pokémon Hunter J and Team Galactic, however he was unable to defeat Saturn in battle. He later monitored the changes around Mt. Coronet caused by Cyrus' attempts to summon Dialga and Palkia.