The Lumiose Gym is a high-tech tower known as the Prism Tower. It is initially closed when the player makes his or her first visit to Lumiose City, as the Tower loses power from a blackout. After the player has defeated Team Flare in the Kalos Power Plant, the Prism Tower regains power and is now able to be challenged.
The gym features a series of guarded elevators. Recognition questions about various Pokémon, asked by Bonnie, must be answered correctly to ascend the tower and challenge Clemont. However, before an answer may be deemed correct or incorrect, the respective elevator guard must be defeated.
Once the player gets the question correct, they will move on to the next floor, and the remaining trainers, if not battled, will be inaccessible. Clemont is at the top (6F), after beating him (or before), the player can use the elevator to leave directly.
Trainers
Trainers are listed in order of the elevator they are guarding.
Ash went to the Lumiose Gym first in Kalos, Where Dreams and Adventures Begin! since the Santalune Gym was closed at the time. As Ash was about to challenge it, he was denied access as he had no other badges from the Kalos region.
It later appeared fully in XY009 Where most of the episode is set. Ash, Serena, Clemont and Bonnie all go to the Gym to receive it back off Clemont's robot that he made. In the end the gym was passed the Clemont's dad while Clemont is travelling.
Trivia
This Gym is tied with the Mossdeep Gym in Emerald Version for the highest amount of common Trainers in a Gym, with both having 12. However, in this Gym the player only needs to defeat four of them in order to get to the Gym Leader.
