You have to got 7 Gyms Badges.
In both diamond and pearl the one city is Hearthome
The sixth gym is in Canalave City of Pokemon Pearl, west of Jubilife City.
You can't go to Celadon city in Pokemon Pearl. Its from an earlier region.
Between pastoria city and hearthome city.
That is the city you start at!
Its not in a city its in a tower.
Jublife city.
i think it is in jubilife city.
south of hearthome city
You cant
Veilstone City
there is no such thing as "sunset city"