there is nobody in the way
you have to beat the elite four
You finish the Pokemon league and get the national pokedex You finish the Pokemon league and get the national pokedex
The fifth gym can be Jasmine or Chuck. No one is blocking any of the gyms. Although Jasmine might not be at the gym until you help the injured Pokemon.
You must have defeated Cyrus at Mt. Coronet and battled Dialga.
You will head to the Pokemon league west of virdian city there you will meet 8 guards asking you for the badges you will need surf on a Pokemon to get trough, later on you come to "victory road" (dungeon) there you need strength on a Pokemon when you get passed that you will need to face the "elite 4(four)" and "Rival" (blue/red/Gary or whatever).
just wait a few days or hours
in pearl u beat hearthomes gym so its probally the same in platnium
by not to tell
you don't get dark rai if you beat the Pokemon league 10 times you have to get the membership card in the event and that event passed already so you have to use the action replay to get him or trade for him
Check your pokedex if you don't have 60 Pokemon in it go start capturing Pokemon. When your done talk to prof oak.
surf to mt pyre, then battle with the adim (Matt i think) then you should be able to get passed (there are guards that weaken you they are easy just bring strong Pokemon (31-higher)
Get max repels for the cave then try getting one Pokemon to learn all Hm's Sneasel works the best to get by eaach door in the elite four