After getting all the badges the 8th gym will open You beat givoanni
OR
Walk Through Walls!!!!!!!!!!
Fight Misty laduh
The last gym leader is the boss rocket to get him to the gym beat team rocket at: Mt moon, Game corner, Pokemon tower, Silph co. After you do this get the 7th badge if you haven't then you can get the last badge in viridian city.
go to the elite 4
The 8th Badge in Pokemon Diamond is called The Beacon Badge.
the boulder badge from brock
earn it.
fuschia.
Fuschia city.
In Vermilion city.
fap
the ground gym
Head to Cinnibar Island for your next badge.