How Do You Become A Gym Leader In Pokemon Platiunm?

You two r wrong it is possible I've done it 1st you finsh the game then start again but when professor rowan askes to complete the pokedex 4 him say no 10 times then he will say want to come gym leader with all your previous pokemon say yes and your gym leader of the town twinleaf

You can't do that you can only be either dawn or lucas of which whom you select at the beginning.

Do you uav 2 erase the previous file 2 do the gym leader thing