At the start of his journey, Gary chose Squirtle as his Starter Pokémon. Squirtle was used to battle Samurai's Metapod. The samurai, despite being defeated, was impressed by its Trainer's strength and prepared himself for future battles against talented Trainers of Pallet Town. An alternative version of Gary was seen choosing Squirtle as his Starter Pokémon.
Gold and Silver series
During the Silver Conference, Gary sent Blastoise during his battle with Ash. Ash's Heracross went to use Fury Swipes, to which Blastoise fired Hydro Pump. Heracross hid behind a rock, to which Blastoise used Hydro Pump to shatter the rock, hit Heracross and defeat it. Ash was forced to send his Muk, who got hit by its Skull Bash. Muk fired a Sludge Bomb, which Blastoise reflected with Rapid Spin, and used Hydro Pump to strike Muk. Ash sent Bayleef, whose Vine Whip was used to toss her away to evade Hydro Pump. She then used Razor Leaf, which Blastoise deflected by using Rapid Spin. However, Blastoise hurled her into the wall after using Bite. In the second half of the battle, Blastoise battled against Ash's Charizard and due to its type advantage, it had the upper hand: its Rapid Spin protected it from Flamethrower. Charizard dodged its Hydro Pump, but suffered its Skull Bash attack.
Charizard flew up, evading Hydro Pump and using Flamethrower on Blastoise, but missed. Ash came up with a very unique strategy, ordering Charizard to melt the battlefield with Flamethrower. When Blastoise used Hydro Pump to cool down the field, Charizard used the steam to get close to Blastoise, preventing it from aiming with its cannons. Blastoise managed to duck underneath Dragon Rage and used Bite on Charizard's shoulder but Charizard ignored the pain and hurled Blastoise into the ground with Seismic Toss. Blastoise attempted to stand up, but the Shellfish Pokémon was defeated by this strike. Ash, during his battle against Harrison, remembered his battle with Gary, including his Blastoise. Before the group parted ways, Team Rocket went to take people's Pokémon at the League. To battle them, Gary's Blastoise fired Hydro Pump to stop them, while Harrison's Blaziken and Ash's Pikachu blasted them away.
Diamond and Pearl series
To prevent Pokémon hunters from capturing Shieldon, Gary sent Blastoise. Its Water Gun, along with Pikachu's Thunderbolt, struck the hunters' vehicle, to which the hunters retaliated by having their Golbat use Wing Attack on Blastoise. When the heroes went to rescue a Shieldon from J, the latter, riding a Salamence, sent Drapion to battle Ash and Pikachu. Drapion used a Cross Poison to wound Pikachu, whereas Gary sent a Blastoise to assist Ash. Blastoise fired a Hydro Cannon on J's Salamance, who retaliated with a Hyper Beam. Pikachu used Thunderbolt, but Drapion endured the attack. It retaliated with Pin Missile, combined with Salamence's Hyper Beam, countering Pikachu's Thunderbolt and Blastoise's Hydro Cannon, and hitting those two Pokémon. As J received report of the police officers coming their way, she canceled the battle and flew away.
Gary used Blastoise to save Ash and Goh from a wild Onix. It used Hydro Pump, causing the angered Rock Snake Pokémon to flee. Blastoise also helped fight against a wild Moltres that Gary needed to fight in order to obtain one of its feathers for Project Mew. Though it was able to put up a fight, even surviving through Moltres's Burn Up attack, Blastoise eventually fainted and was switched out for Electivire.
With Professor Oak