Gary received Blastoise as the Squirtle from Professor Oak when he chose his starter to start his adventures. Ash wanted the Squirtle that Gary received, but ended up with Pikachu. Blastoise was first seen in a flashback when it was a Squirtle, where it battled and defeated Samurai's Metapod. However, it was not revealed which trainer the Squirtle belonged to and it wasn't until the Silver Conference that Blastoise was finally revealed to be Gary's Starter Pokémon. Brock and Misty were both aware that Blastoise was Gary's Starter Pokémon; presumably Brock had battled it while it was a Squirtle when he was the Pewter City Gym Leader, although how Misty was aware that Blastoise was Gary's Starter is unknown since she was travelling with Ash by the time Gary would have reached the Cerulean City Gym.
During the Silver Conference, Blastoise was the most powerful Pokémon Gary used during his battle with Ash, defeating Ash's Heracross with a single Hydro Pump, forcing Ash to recall his Muk and knocking out Bayleef by hurling 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 until Ash used 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, defeating the Shellfish Pokémon.
Blastoise is one of Gary's most loyal Pokémon, and traveled with Gary when he became a Pokémon researcher. Along with Umbreon and Electivire, it was often used while Gary was in Sinnoh.