In The Isle of Armor DLC for Pokémon Sword and Shield, you unlock an island outside the border of the Galar region where you can find a variety of new Pokémon available to you. You can start the journey almost anytime in Pokémon Sword and Shield after you open the Wild Area and have purchased the expansion pass. During the journey, you will have a chance to pick up a Squirtle or a Bulbasaur that have a Gigantamax form you can use in a battle.
To have the option to choose between these two Pokémon, you will need to progress through the DLC’s story. The opportunity to pick a Bulbasaur or Squirtle with the ability to Gigantamax happens after the first trial at the Master Dojo. The leader, Mustard, will give you the challenge after three Galarian Slowpoke steal the uniform of Klara or Avery. You must venture out into the Wild Area to battle them, and steal the articles of clothing back.
After you retrieve all the articles of clothing, return to the Master Dojo to receive your prize. Honey will summon the Bulbasaur and Squirtle available to you. Unfortunately, you can only pick one of the two. If you want to receive both, you will have to trade for the Pokémon you did not pick from another player, or by having The Isle of Armor DLC with another copy of Pokémon Sword and Shield.
You will need to evolve your chosen Pokémon to its final form to unlock its Gigantamax potential to use it in combat against other trainers.