Super Mario Bros. 3
Gargantua Koopas first appear in Super Mario Bros. 3 where they can be found exclusively in Big Island. Aside from them, Grand Goombas and Piranhaci Gigantici, they all reside on Big Island trying to attack Mario or Luigi. Like normal Koopas, they hide in their shells when jumped on and can be kicked around too.
Super Mario Galaxy 2
Gargantua Koopas reappear in Super Mario Galaxy 2 as enemies found in Supermassive Galaxy. These enemies are only in three on a small planetoid in the galaxy. They can only be defeated by being drilled from underneath their bellies. These are related to Giant Koopas.