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.
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Gold, Silver, Crystal, HeartGold, and SoulSilver Versions. They all have 16 gyms (two regions).
16
Pokemon HeartGold, SouldSilver
you keep on battling the gyms
you have to beat the first 7 gyms and team rocket at goldenrod city. then the man will let you pass.