At this time that cannot yet be done.
Generation 8 has not yet been implemented. Only the Galar starters and Mr. Rime from a previous event are in the game.
Gen 8 Pokémon, Gyms and everything else that belongs to it will be added in a future update. When exactly this will happen we don't know.
With the V5 update, this is all possible now.
You can fight the Galar Gyms on the new maps.
Once you've beaten all 8, you can encounter the Legendary Pokémon from Galar.