Grass that's Erika's gym
The 8 Gyms in Leaf Green are in Pewter City, Cerulean City, Vermilion City, Celadon City, Saffron City, Fuchsia City, on Cinnibar Island and in Viridian City.
You defeat him in Celadon at his hideout then at Saffron, then when you finish the rest of the gyms and help Bill then go back to Viridian City, and then he will be at the end of the gym waiting for you to battle his Ground-type Pokemon.
Viridian city, Pewter city, Cerulean city, Vermilion city, Celadon city, Fuchsia city, Saffron city, Cinnibar island.
You have to beat all 16 gyms.
From the first gym to last it goes: Pewter City, Cerulean City, Vermilion City, Celadon City, Fuchsia City, Saffron City, Cinnibar Island, Viridian City.
Just beat all the gyms. The first one is in Violet City
One is in viridian city, Another in pewter city, Another in cerulean city, another in vermilion city, another in celadon city, another in fuchsia city, another in saffron city, and another at cinnibar island.
Vermillion, Saffron, Cerulean, Celadon, Fuchsia, Pewter, Cinnabar, Viridian
Battle the Eight Gym leaders. It is just like Johto.
Viridian city, Pewter city, Cerulean city, Vermilion city, Celadon city, Fuchsia city, Saffron city, Cinnibar island.
Viridian City, Pewter City, Cerulean City, Vermilion City, Celadon City, Fuchsia City, Saffron City, Cinnibar Island.
The answer is... No there are no and i repeat NO hidden gyms on Pokemon soulsilver (but there are more gyms in kanto which you can get to by the boat pass from prof elm after you defeat the champion)