you must beat the eighth gym leader and he will give you the badge.
by beating the gym leader
in the 1st gym badge
You can use the GTS in Pokemon Platinum after you get the first gym badge from the Oreburgh City gym.
The fifth gym leader in Pokemon platinum is crasher wake but you get the fourth badge.
Hearthome's badge
Get the first gym badge
if you are looking for the third gym badge i suggest Veilstone city.
umm...you beat the 7th gym
it's the 3rd badge!
The third Gym is the Gym in Veilstone City. You can get there by looking on your map. The leader specializes in Fighting Pokemon.
In Diamond, Pearl and Platinum 7th gym is in Snowpoint Town.
Go to the library top floor