Go to driftveil. Go up, then climb the stairs. Go left. If it's blocked,you need to beat team plasma grunts that are doing something. I don't have my game on me so I can't double check, but I hope this helped.
The 5th gym is the Fuchia city gym Gym leader Koga
in the museum
The 5th gym leader in Johto is Chuck. He is located in Cianwood City.
it is at driftveil city
Lenora, the second gym leader, is found in her gym AKA the museum in Nacrene City.
Driftveil City. Gym Leader Clay!
Have a water ice or grass pokemon.
pass the 5th gym leader
The 5th gym is the Fuchia city gym Gym leader Koga
in the museum
Fantina
charizard
In sapphire, the 5th gym leader is your dad, in that city after Oakdale.
You Cant
Herdier and Watchog.
The 5th gym leader in Johto is Chuck. He is located in Cianwood City.
The 8th gym leader is Drayden, who uses Dragon-type Pokemon. You face him in Opelucid City.