every second gym leader will allow you to use higher level Pokemon
For each gym leader you beat the trick master creates a new level. The last level is after you beat the elite four.
Gym Leader Elesa has two level 25 Emolgas and a level 27 Zebstrika.
In Pokémon Emerald, the 6th Gym Leader, Winona, the Gym Leader of the Fortree City Gym, the Pokémon she has are a Level 29 Swablu, a Tropius at Level 29, a Skarmory at Level 31, a Pelipper at Level 30 and an Altaria at Level 33.
hakunamata
For the second gym leader you need about three to five Pokemon and the recommended level is 15-20.
Nosepass level 11.
50's
the mayer
Burgh has level 21 Whirlipede, level 21 Dwebble and a level 23 Leavanny.
the piraka dont have a leader they mainly hate each other :) But Zaktan is usually considered the leader.
There was no single leader during WW1. Each country had their own leader.