The closest to battle is cianwood gym but if you have finished that it will be mohagany town gym
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It is on Cianwood City Island. You have to surf there from Olivine City. (NOTE: the gym in Olivine is actually the 6th Gym battle so go to Cianwood Gym first) Before you go to Cianwood City make sure you visit the Olivine lighthouse top floor to talk to the Olivine Gym Leader about a sick Pokemon first.
you go back to olivine
In Cianwood City you receive the medicine from the medicine shop. To give the medicine to the gym leader you have to go to the top of the lighthouse in olivine, which is where she will be. She will then return to her gym where she will battle you. Be careful - her Pokemon (2 magnemites and a Steelix) can be tough! Good luck! :)
Go to the Cianwood Pharmacy by surfing to the West of Olivine. The man gives you the meds, fly or surf back to Olivine and head back up to the top of the tower.
You can get it in the house south east of the olivine gym. HOPE THIS HELPS! :)