In order to unlock Legendary Pokémon encounters, a player must obtain ribbons by defeating gym leaders. Each ribbon represents a Pokémon region and unlocks its respective legendaries. All gym leaders in a region need to be defeated. You can track your progress from your profile by checking your obtained badges. In the past, elite 4 members, champions, battle frontier brains, and battle maison members had to be completed for all legendary Pokémon to become obtainable but is no longer the case after a v4 update.
Note: Unlike past gyms which can be accessed directly via the site navigation, Galar gyms are located in the towns of the live maps introduced in v5 of Pokémon Vortex.
Unlocked with Galar Ribbon