Well you have to have past the distortion world and then go challenge the other gym leaders after that you have to meet one of the elite four that tells you to beat teh 8 gym leader
How Do You Get To The Eighth Gym In Pokemon Platinum?
Go to snow point city after catching/defeating Giratina.
You have to find giratina and battle him
They r right but actually getting in is a completely different matter
The guy is still there
Ok, first of all you have to look through Mount Coronet and find Cyrus and team galactic. After you do some stuff the guy blocking Sonny shore city will be gone and you will be on your way! (I've beat the game five times:)
You have to capture giratina first and then the guy at the gate will let you in.