Serenity Isle's Gym is the second in order of Tunod region - although you can skip it untill later. It's the Ground-type specialist gym. Beating it gives you the Dirt Badge.
* - Rhyhorn is present on earlier betas of Glazed, while Vibrava appears on latest ones
Order to reach Terry: enter the gym, battle Antonio and drop at the bottom hole, the go up and right; don't go to the ladder at front, instead take the left side and then go up at the corridor; go up untill you reach Terry (you'll have to battle Oliver)