To get a Gigantamax Pokémon in Pokémon Go, you first need to defeat it in a Max Battle at a Power Spot. Afterwards, you'll have a limited number of Premier Balls to catch the Gigantamax Pokémon.

If you want to Dynamax it again, then get on filling that ... have their own unique Gigantamax form despite technically being the same Pokémon. In Pokémon Sword and Shield, Gigantamax Pokémon ...