You have to get some medicine for the Pokemon she is looking after :)
you have to surf to cianwood city,and get the medicine from the medicine house.
In the light house! In the light house!
To get in the 8th gym you have to go find the gym leader in the light house.
you can get out the gym leader on the lighthouse is bring her a secretpotion that you can get to the cianwood pharmacy
yo need to get the secret potion from cianwood city and give it to ampharos
the gym leader is now at gym
Well the gym leader will not be there if you do not get the secret potion. if you are talking about beating the 6th gym ( fighting type) the girl on top of the light house is the gym leader
she is in the light house in olivine city.
The medicine is in Cyanwood city.
The 6th gym leader is in Fortree City.
you got to the top of the sunnyshore light house and talk to the blond guy he is the gym leader then you can face him.
The 6th gym leader is in Canalave City.