Ballonlea Stadium is the largest building in the town and is the site at which the fifth installment in the Galarregion's Gym Challenge takes place. It is located at the northernmost part of the town, across a wooden bridge crossing a small lake. Its exterior is primarily gray in color, with purple accents to signify the Fairy type. Like most Stadiums in the region, the Ballonlea Stadium contains a Poké Mart and a uniform shop.
The Gym mission is based on the premise that Gym Leader Opal is searching for a person to take her place as the Gym Leader so she can retire. In order for Opal to select a preferred candidate, challengers are made to undertake an examination as their Gym mission. The mission consists of three Trainer battles. At the end of each battle's first turn, the player is asked a question with two possible answers. Answering correctly results in a boost to some of the stats of the player's Pokémon, whereas answering incorrectly results in a stat reduction.
While pursuing Sordward and Shielbert, the player, Hop, and Piers will arrive at Ballonlea Stadium, only to learn that Bede has already taken care of most of the Dynamax Pokémon the brothers let loose in the Stadium. Before the player's group can leave, Bede demands a rematch with the player. After some assurance from Opal, the player will accept the challenge. After being defeated, Bede leaves to deal with the remaining Dynamax Pokémon at the Stadium.
To everyone's surprise, Ash's overly decorated cake ended up winning. Opal then challenged Ash to a battle, using her Alcremie against Ash's Pikachu. A while into the match, Opal Gigantamaxed her Alcremie and had it use G-Max Finale. Pikachu seemed to Gigantamax as well afterwards, but it turned out that he had simply grown giant due to eating the immense amounts of cream contained in Alcremie's G-Max Move, rendering him too fat and full to continue battling, so the match was called off. Later, everyone ate the cakes decorated during the contest.
Before entering the Stadium, Opal asks each challenger a quiz question about her age. Depending on their answer, their Pokémon's Attack and Special Attack stats either increase or decrease drastically at the beginning of the battle.
After Hop had won his battle, Henry's turn came. Despite his Sirfetch'd's stats getting lowered at the beginning of the battle due to him having answered the quiz wrong, he was still able to defeat Opal's three Pokémon without losing any of his own ones, using an Expert Belt he had received from Sonia to overcome the stat reduction. During the battle, Opal also made a public announcement of her plan to retire as a Gym Leader once the year's Gym Challenge was over.
In PASS18, Casey was the next challenger to take on Opal. However, despite using her GigantamaxToxtricity against Opal's Gigantamax Alcremie, Casey was still defeated, ending her run in the Gym Challenge.
In PASS20, Bede visited the Gym, following an invitation from Opal. She took a liking to Bede's personality and deemed him as a fitting successor for her. Afterwards, Bede started training at the Gym in preparation for taking on the Gym Leader position.
Pokémon used in the Gym
Alcremie ↔ Gigantamax Alcremie ♀
Alcremie is Opal's first known Pokémon. She was first used in Opal's Gym battle against Henry, where she bombarded his Sirfetch'd, Lancelot, with a barrage of high-calorie cream missiles. After surviving Alcremie's attacks, Lancelot defeated her with a Max Steelspike boosted by an Expert Belt.