This is a Wild Area event held by Sword and Shield. This Max Raid event was distributed to players to commemorate the Pokémon Day 2021.
This Max Raid event adds Gigantamax Pikachu as a possible encounter into the previous Max Raid encounters.
This event also has an increased chance of landing a Shiny Gigantamax Pikachu; players have a 2% chance of landing on a guaranteed Shiny, only at the 5 star tier.
Besides that, there are also specific rewards for the amount of times Max Raid Battles against Gigantamax Pikachu were won:
1,000,000 battles won: players receive 100 Armorite Ore and 100 Dynite Ore.
To import these files into your save, use PKHeX's block importing function. Our tutorial should guide you through it.
Our first post of this page contains some data relevant to the importing stage.
There's info and some common questions regarding Wild Area Events here.
To see other Wild Area Events, here.