There is no key for the 8th Gym. You have to look for Wallace in order to access the gym. He's located in the Cave of Origin, which is directly north of the gym.
the ground gym
you go to sootopolis city
Beat the gym leader
Beat the 8th Gym leader.
juan
evergrande city!
The eighth Gym Leader, Juan, can be found in the Sootopolis City Gym after you have dealt with Teams Aqua and Magma. He trains Water-type Pokémon.
You have to face the Elite 4 after you beat the eighth gym leader. The game gets a little better & less boring when you do.
Find the cave with the broken sumerine and use dive to get up and battle the team auqa people and go in the last entrance in the cave and the key should be there.
Go to the Pokemon league and become the champion
You need to step on every piece of ice one time. Then the staircase appears.
first of all you get groudon not rayqaza that's emerald and you get him near the eighth gym