Gigantamax Forms are a variant of Pokémon that occur while the Pokémon has activated its Dynamax, but only on special occassions when these Pokémon were caught in Max Raid Battles. These changes are mostly appearance based but do come with a unique changs. Each gains access to a special G-Max Move which has a bigger effect than standard Max Moves.
Some of the Gigantamax Forms are exclusive to certain games but can be obtained in the other game if you do the Max Raid Battle with a friend who has that game. There will also be Wild Area Events that will spawn more of the Gigantamax variant away from the usual locations
Some new Gigantamax Forms are also being added through the Expansion Pass. The Isle of Armor also introduces a new feature of Max Soup. If you create the Max Soup and feed it to a Pokémon whose species is capable of Gigantamax, then it too will be able to Gigantamax