Gym Leader

A gym leader is the strongest trainer in the gym. They have a boost of 15% starting at Johto (Hoenn has a 20% boost, Sinnoh has a 30% boost and Unova has a 40% boost). Also if you beat a gym leader you can get gym badges. If you collect all of them in a region you will be able to fight the Elite Four in that region. Then, move on to the next region.

Gym Leaders
Brock is a Rock-type specialist. He is the leader of Pewter Gym, being the first gym leader a player can face.

Beating Brock at Pewter Gym will award the Boulder Badge, which allows the player to progress to Route 3.

As Pewter Gym Leader
Misty is a Water-type specialist. She is the leader of Cerulean Gym, being the second gym leader a player can face.

Beating Misty at Cerulean Gym will award the Cascade Badge, which allows the player to progress to Route 5 and also allows a player to talk in chat.

As Cerulean Gym Leader
Lt Surge is an Electric-type specialist. He is the leader of Vermillion Gym, being the third gym leader a player can face. Beating Lt Surge at Vermillion Gym will award the Thunder Badge, which allows the player to progress to Rock Tunnel.

As Vermillion Gym Leader
Erika is a Grass-type specialist. She is the leader of Celadon Gym, being the fourth, fifth, sixth or seventh gym leader a player can face. Beating Erika at Celadon Gym will award the Rainbow Badge, which allows the player access to Saffron City.

As Celadon Gym Leader
Koga is a Poison-type specialist. He is the leader of Fuchsia Gym, being the fourth, fifth or sixth gym leader a player can face, and also the third member of the Johto Elite Four. Beating Koga at Fuchsia Gym will award the Soul Badge, which allows the player to use Surf outside of battle.

As Fuchsia Gym Leader
Sabrina is a Psychic-type specialist. She is the leader of Saffron Gym, being the fifth, sixth, seventh or eighth gym leader a player can face. Beating Sabrina at Saffron Gym will award the Marsh Badge.

As Saffron Gym Leader
Blaine is a Fire-type specialist. He is the leader of Cinnabar Gym, being the fifth, sixth or seventh gym leader a player can face. Beating Blaine at Cinnabar Gym will award the Volcano Badge, which allows the player to challenge the Viridian Gym.

As Cinnabar Gym Leader
Giovanni is a Ground-type specialist. He is the Team Rocket boss, as well as the leader of Viridan Gym, being the sixth, seventh or eighth gym leader a player can face. Beating him at the Viridian City Gym awards the Earth Badge.