In Pokemon Rejuvenation you face the challenges of 18 Gym Leaders, one of each type. Currently there are 12 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.