To open Sootopolis Gym in Pokémon Omega Ruby, you need to first defeat the Elite Four and become the Pokémon Champion. After that, head to Sootopolis City, where you will find the gym entrance blocked by a large body of water. You must complete the storyline involving the Legendary Pokémon Kyogre and Groudon, after which the water will recede, allowing you to enter the gym and challenge its leader, Wallace.
... No
After visiting Sea Mauville in Pokémon Omega Ruby, you typically continue your journey by heading back to Route 108. From there, you can explore the ocean routes and challenge the remaining Gym Leaders, including the Sootopolis Gym. Additionally, you can engage in battles with trainers and seek out new Pokémon to catch as you prepare for the Pokémon League.
Go into the cave of origin fight him and let him faint or catch him and when you come out go to the gym
In Ruby and Sapphire, you have to fight the respective legendary before the doors open.
You might need to open the door to the 8th gym in Pokémon Ruby, Sapphire and Emerald. To do this you first have to awaken Rayquaza.
The eighth gym leader is in the underwater city(forget the name) but it is where you fight groudon. after you beat him the gym should be open.
go to map land 6 but you must beat all gym leaders including the elite four
Pokemon ruby where is the 6 gym
You must finish the mission that would make the weather the same again. After that, everyone will be all happy and the eighth gym will be open to you.
Norman's gym
To reach Mossdeep City in Pokémon Omega Ruby, you must first obtain the HM Surf, which allows you to navigate water. After defeating the Gym Leaders in Sootopolis City, head east to Route 127, then continue to Route 128. Follow the water route to the north, and you'll eventually arrive at Mossdeep City. Be sure to explore the area and challenge the local Gym for more adventure!
its the petalburg gym and/or gym 5.