This is a Wild Area event held by Sword and Shield.
This event allows many Gigantamax Pokémon to spawn as event boss in 5-star raids.
This event was available from November 1st until the end of online services for Pokémon Sword and Pokémon Shield.
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 would contain 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.