Starting February 19th, 2021, players that visit GameStop and access the Pokémon Pass app on their mobile devices can collect a free serial code for a shiny Toxtricity.
It was not indicated on the poster that it has Gigantamax. (Not that it truly matters, just give it Max Soup)
Form for this is also not known. Unknowns include, if only one form will be distributed, if form distributed correlates with version, etc.
While Pokémon Pass will only give out serial codes to those that visit GameStop stores in the US, since the Switch isn't region locked, the codes can be claimed by anyone across the world.
Image:
Source: https://twitter.com/iceposs2000/status/1358278373764603906