First you must find the underwater cave where groudon is and beat the team magma leader to send groudon to the cave of origin in sootopolis. Then you go back to sootopolis and you meet steven and the eighth gym leader Wallace up the path just to the right of the pokemart. Next you enter the cave of origin and battle groudon deep inside (save right after entering the room groudons in because you are taken to battle groudon when you get about five steps towards him). Try to catch groudon without using your master ball and exit the cave. After that, go to the gym to talk to steven outside of it and then he leaves and you can enter the gym. GOOD LUCK!!
Wassilij
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.
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.
The eighth gym leader is in Viridian City. It is the team rocket's leader!
in sootoplis
The eighth gym leader is Claire in Blackthorn City
beat wallce
Sootopolis City. Check www.serebii.net for more info on Pokémon Ruby.
Defeat the eighth gym leader.
Catch groudon
If you mean "Is Juan the leader in Pokemon ruby?", then no. It's Wallace who's the leader. Juan is leader only in Pokemon emerald. The champion is Wallace in Emerald, but Steven is the champion is Ruby and Sapphire.
After obtaining the eighth badge in Pokemon Ruby, players should continue to combat the Elite Four. After beating the Elite Four, the main storyline of the game is complete.