Assuming you mean after Koga (as Sabrina can be taken on about the same time). Head to Saffron, if you've not given the guards the tea from Celadon (in the mansion) do that first that will open the gates for you to get into Saffron, you will find Saffron overtaken by Team Rocket, head to Silph Co.
If you've beaten Sabrina as the 5th. Head South from Celadon via the Cycle Road or alternatively head South from Vermillion via Routes 12-15. (or straight south from Lavender). They will both lead you to Fuchsia and require you to have gotten the Pokeflute from the Lavender Tower as both have Snorlax in the way.
fuschia.
fap
On the 5th floor of silph co.
it is at driftveil city
The 5th gym is the Fuchia city gym Gym leader Koga
Koga, he is in fuschia city, he uses poison Pokemon
it is given to you when you defeat the 5th gym leader by his wife
you must defeat the 5th gym and get surf in safaria zone the go to Pallet Town and surf downwards.
well you will have to defeat the 5th gym leader and once you defeated her she will give you the TM move shadow claw you will find the 5th gym leader at hearthome city before you deafeat the gym leader you will need to defeat the 4th gym leader in order to defeat the 5th gym leader
fuschia.
fap
When you defeat the 5th gym leader and when you leave the gym head to the left and a woman will give you the HM
the 5th gym is saffron gym with sabrina!
you have to defeat the gym leader in the 5th gym or something ...
On the 5th floor of silph co.
charizard
u have to defeat the 5th gym leader in cianwood city.and then talk to the women just outside of the gym and she will give u hm fly.