Gigantamax Cinderace is only available in The Isle of Armor expansion, available in the Expansion Pass DLC. You can unlock Gigantamax Cinderace on the Isle of Armor by feeding any Cinderace in your party or box Max Soup.

Players can get a Gigantamax Cinderace in Pokemon Sword and Shield by feeding Max Soup to a regular Cinderace. The Isle of Armor DLC for Pokemon Sword and Shield introduced the idea of Max Soup. Max Soup allows players to toggle whether a Pokemon will be able to Dynamax or Gigantamax.

The three starter Pokémon in Sword and Shield now have the opportunity to receive new Gigantamax forms through the DLC expansion, The Isle of Armor. You won’t be able to unlock these new forms at the start of the new region. You can only unlock them after you complete the second part of the story and learn about Max Soup.

Getting these big ‘mons for yourself will involve exploring the Wild Area and taking on Max Raid Battles. If you encounter one of the species listed below, battle it, and manage to catch it, there will be a chance that it will be capable of Gigantamaxing.