Gary and Ash used to be friends, but Gary became Ash's rival as a child when they broke an old Poké Ball and both wanted to become Pokémon Trainers. At the age of 10, Gary recieved a Squirtle from his grandfather, Professor Oak. He then had a cheerleading group follow him with a limosine. Eventually, Squirtle evolved into a Blastoise and Gary captured over 200 Pokémon.
He watched Ash beat Team Rocket in Showdown at the Poké-Corral, and later advanced to the Top 32 in the Pokémon League. However, he lost to a powerful Pokémon Trainer called Melissa and was forced to leave the Pokémon League. This also made him realize he needed to change himself. He returned in A Tent Situation, where he defeat Team Rocket's Pokémon with his Nidoqueen. He then had his first battle with Ash and won using his Eevee. He then leaves for the Johto region.
Gary rents a Hoothoot from Hagatha and makes his way through a forest, all the way to obtaining 8 more badges and entering the Johto League. His Eevee also evolves into an Umbreon and he enters the Silver Conference. He advances to the Top 16 in the Conference, and he used his Nidoqueen, Magmar, Arcanine, Scizor, Golem and Blastoise against Ash. However, Ash beat him when Charizard defeated Blastoise.
Gary then decided to give up on being a trainer and become a researcher instead. He gave Ash the Poké Ball, but when Gary left all his Pokémon but Blastoise at Professor Oak's Laboratory, Ash returned it to him and Gary left.
Gary joins Dora and Jared on Sayda Island on a new project: reviving extinct Pokémon. Gary infuses a fossilized Pokémon Egg with an Old Amber he found in Grampa Canyon, reviving an Aerodactyl. Professor Oak and Tracey arrive on Sayda Island and try to stop it from crossing the sea and wreaking havoc on the people in town. However, it is soon revealed it doesn't want to leave. Gary then uses his Umbreon and Dodrio to stop Butch and Cassidy from taking it.
Since the very start of his own journey, Gary has shown exceptional skill as a Pokémon Trainer, catching and training an astonishing number and variety of strong Pokémon in a short period of time, losing battles very infrequently. However, since retiring as a Pokémon Trainer to become a researcher, it isn't known if he continues to catch new Pokémon often.