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