In Pokemon Rejuvenation you face the challenges of 18 Gym Leaders, one of each type. Currently there are 7 Gyms to face in the current update of the game.
Each Gym Leader has 4 Pokemon in Casual Mode and 6 in Normal and Intense. They also have different Fields where you battle them on, which helps their Pokemon out quite a bit. After you defeat them, a Badge will be awarded to you, allowing you to use a HM and/or increase your level cap.