Gary Oak, Ash Ketchum's original rival from Pallet Town, evolved from Pokémon Trainer to researcher by the Diamond & Pearl era. This shift followed his defeat by Ash at the Johto League's Silver Conference, where he publicly ended their rivalry, presenting half a broken Poké Ball as a symbol of renewed friendship. He kept only his starter Blastoise, leaving other Pokémon with Professor Oak to pursue science.

In Sinnoh, Gary assisted Professor Rowan, specializing in fossil Pokémon research and ecosystem preservation. He intervened to protect Shieldon from Pokémon Hunter J, coordinating with Ash and friends to rescue the captured fossils. During this mission, he deployed his newly acquired Sinnoh Pokémon, Electivire, against J's forces. His research included evolutionary biology; he offered Ash a Razor Fang to evolve Gligar into Gliscor but respected Ash's choice for natural progression.

Gary later helped thwart Team Galactic's attempt to capture the lake guardian Azelf. Though unable to prevent the abduction, he informed Professor Rowan and retrieved the Adamant and Lustrous Orbs after Team Galactic's defeat. His demeanor showed significant maturation: while occasionally teasing Ash, he consistently offered strategic advice and assistance, openly criticizing Team Rocket's recurring interference as "lame."

Post-Sinnoh, Gary joined Project Mew to gather data on Mythical Pokémon. His missions involved collecting a Moltres feather through a coordinated battle with Ash and Goh against Moltres, and capturing the Legendary Regidrago during a trial mission. He also led a raid against Articuno using Tyranitar, Umbreon, and Arcanine, though the effort failed against the Legendary's overwhelming power. He initially questioned Goh's suitability as Ash's companion but later acknowledged Goh as a rival within Project Mew.

His attire reflected his research focus, typically featuring a black leather jacket, white shirt, khaki cargo pants, and black shoes, marking a departure from his earlier flamboyant Trainer clothing.