Brock is the first gym leader in pewter city.
The 7th gym leader in Pokemon yellow is in Cinnebar Island, which is south of Pallet Town. You need a Pokemon with Surf to get there.
Giovanni
i cant explain
celadon city
The 7th gym leader is Blaine. He the gym leader of Cinnabar Island gym. He uses Fire type Pokemon and he will give you the TM that contains Fire Blast when you defeat him.
There are 8 gym leaders, you have to specify which Gym leader you're talking about. If you beat the 1st one, you have to travel east until you reach a cave called MT. Moon.
The first gym in Pokemon diamond is in Oreburgh City
You kill the Gym Leader's Pokemon with your Tepig.
In the town where Ltg. Serge is a gym leader there is an officer there with a squirtle. Beat the gym leader and talk to her afterwards.
The 5th gym in Pokemon Yellow is the Fuchsia City gym. The gym leader is Koga who specializes in Poison/Bug type Pokemon. Once won you will recieve the Soul badge, TM06 - Toxic, & be able to use HM03 Surf out of battle.
From the 1st gym leader
Vermillion city (after beating the gym leader)