Wassilij
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.
in sootoplis
The eighth gym leader is in Viridian City. It is the team rocket's leader!
The 8th gym leader is Drayden he uses Dragon type pokemon
The 8th gym leader is in Viridian City.
Catch groudon
Defeat the eighth gym leader.
In the 8th gym. he is the gym leader
volkner
The 8th gym leader is in sunny shore city
the leader of team rocket
in sunyshore city