Platinum Gym Leaders & Elite Four
Platinum's gyms feature several changes from those in Diamond/Pearl. Gyms 3-5 switch their orders and most change their Pokémon to take advantage of the updated local dex. Now the gym leaders only use Pokémon of their specialist type - for example in D/P, Volkner used a Water type and a Normal type in addition to his two Electric Pokémon.
After beating the Elite Four and completing the Stark Mountain prologue, you can rematch gym leaders and the five trainers you partnered with during the game, at the Trainer Café in the Survival Area.
Gym #1, Oreburgh City

Roark
Coal Badge
Rock type Pokémon

Roark - rematch
Rock type Pokémon

Aerodactyl
Level 62
Rock · Flying
Gym #2, Eterna City

Gardenia
Forest Badge
Grass type Pokémon

Gardenia - rematch
Grass type Pokémon
Gym #3, Hearthome City

Fantina
Relic Badge
Ghost type Pokémon

Fantina - rematch
Ghost type Pokémon
Gym #4, Veilstone City

Maylene
Cobble Badge
Fighting type Pokémon

Maylene - rematch
Fighting type Pokémon
Gym #5, Pastoria City

Crasher Wake
Fen Badge
Water type Pokémon

Crasher Wake - rematch
Water type Pokémon
Gym #6, Canalave City

Byron
Mine Badge
Steel type Pokémon

Byron - rematch
Steel type Pokémon
Gym #7, Snowpoint City

Candice
Icicle Badge
Ice type Pokémon

Candice - rematch
Ice type Pokémon
Gym #8, Sunyshore City

Volkner
Beacon Badge
Electric type Pokémon

Electivire
Level 50
Electric

Volkner - rematch
Electric type Pokémon

Electivire
Level 65
Electric
Elite Four #1

Aaron
Bug type Pokémon

Aaron - rematch
Bug type Pokémon
Elite Four #2

Bertha
Ground type Pokémon

Bertha - rematch
Ground type Pokémon
Elite Four #3

Flint
Fire type Pokémon

Flint - rematch
Fire type Pokémon
Elite Four #4

Lucian
Psychic type Pokémon

Lucian - rematch
Psychic type Pokémon
Champion

Cynthia
Mixed types

Cynthia - rematch
Mixed types
Other trainers

Cheryl
Survival Area
Mixed types

Buck
Survival Area
Mixed types

Marley
Survival Area
Mixed types

Riley
Survival Area
Mixed types

Mira
Survival Area
Mixed types