You must go to Viridian city at the gym that is locked will be open again after you complete all the other gyms.
Beacon Badge
The eighth badge can be obtained by defeating the gym leader in viridian city.
yes ash will get his 8th gym badge
you must beat the eighth gym leader and he will give you the badge.
the ground gym
Sunyshore City
You can obtain the 8th gym badge (the earth badge) in fire red from Giovanni in Viridian City.
You have to beat the eighth gym leader to get to the Elite Four. Without defeating the eighth gym, you cannot get the S.S. Aqua ticket. You also cannot capture Lugia without the eighth badge.
beat the league
Beat the 8th Gym leader.
beat the #?*@! gym leader
Beat the gym leader