Galarian Forms can't Gigantamax. Along with the new Galarian Forms there will be new Pokémon introduced such as the Steel and Dragon Type eeveelutions, Chromeon and Wyvereon, a Haunted set of armor who's Ghost/Steel, The Water/Fire teapot Elephant Rose is seen having, Wooloo's evolution, a dark type wolf in sheep's clothing, and more.
Your starters can Gigantamax, and only Pokemon that are at their final stage can Gigantamax. The fat Pikachu and Meowth aren't true.
The actual Evil Team is Team Royal, and they act as knights helping you on your journey. Their true goal is to harness the power from Eternatus and Meteodyna. Oleana is the leader. The player uses Zamazenta or Zacian respectfully to combat against Team Royal during the climax.