i am pretty sure it's strength.
The sixth gym leader,Sabrina, is in Saffron City.
I think you must beat Erika the fourth gym leader in Celadon City
beat the gym leader
the gym leader
In Fuchisa city
The seventh gym leader
The 6th gym is in Saffron City the gym leader is Sabrina she uses Psychic type pokemon.
yes after you beat the second gym leader
beat the shliph co. office biulding
You must beat team rocket at silph co before you can enter the 6th gym.
in fuchsia city
well the answer is quite simple you beat the gym leader duhhhhhhhhhh