beat the first 7 gyms and it opens . Hope i helped :)
First you need to beat Giovanni at the celadon game corner and silph co then get the 7 badges then you can go inside the gym.
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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.
In Ruby and Sapphire, you have to fight the respective legendary before the doors open.
You can find the key to unlock the door in Pokemon mansion
There's a key. You need to go into the mansion on the island, it's in a room in the basement that's difficult to get to... after you get it you can open the door to the gym.
You're going to have to go on the S.S. Anne and then get HM Cut from the Captain. Then you will be able to cut the tree that blocks the way to the gym.