Pikachu is one of only twenty-seven Pokemon that have a Gigantamax Form in Pokemon Sword & Shield. However, unlike other Gigantamax Forms that are obtained by catching that specific Pokemon from a Max Raid Battle, Gigantamax Pikachu is obtained in a special method. In order to receive Gigantamax Pikachu, you must have a Let's Go Pikachu Save File on the Nintendo Switch you are playing Pokemon Sword or Pokemon Shield on. If you do, you will have the ability to receive a special Pikachu (one that cannot evolve) that will have the ability to be Gigantamaxed during battle.
If you have a Save Data for Let's Go Pikachu on the same Nintendo Switch you are playing Pokemon Sword & Shield, you will receive a special Pikachu from a man and his children within the Meetup Spot in the Wild Area. This Pikachu will come at Level 10 and, as mentioned, has the ability to Gigantamax when in battles that allow it. However, it cannot be a Shiny Pokemon and it cannot evolve.
We have questions and answers related to this topic which may also help you: