It shouldn't have been too long since you last visited this stadium. Anyway, approach the staff member at the reception desk to begin the Gym mission.
Know Your Match-Ups: Fire
Fire-type Pokémon are weak to Water, Rock and Ground. They are strong against Grass, Bug, Ice and Steel Pokémon, while they resist Fire, Grass, Ice, Bug, Steel and Fairy moves.
Your best bets are Water or Rock-types. If you chose Sobble, you're sorted. Otherwise, you've had ample chance to find some Water-types. But if you're struggling, you can catch a Drednaw in Galar Mine No. 2. Then go to the café owner in a Pokémon Centre to relearn Razor Shell.
Your third Gym mission is to get 5 points by catching or defeating wild Pokémon. You'll receive 2 points for every Pokémon caught or 1 point if you defeat it instead. The catch is that you'll always be fighting alongside an opposing trainer. Thing is, unless we missed something, there's no penalty to defeating all of the opposing trainer's Pokémon...
Anyway, the area will be divided into three sections. Towards the north, you can encounter the Ghost/Fire Litwick. Over to the west, you can find the Fire-type Vulpix. Finally, to the east, you can run into the Fire/Bug Sizzlipede. At this point, they're all pretty rare Pokémon (Vulpix for Shield players), and you get to keep what you catch!
We're pretty sure there's no way to fail this mission. Even if you exhaust all of the initial wild Pokémon, more will respawn after the 2nd fight. Obtain 5 points and the mission will come to a close. You'll be transported to the familiar place, at the back of the entrance tunnel. Once you're prepared, step forward to meet the Gym Leader and the crowd head on.
Gym Leader Kabu
Kabu begins the battle with two pure Fire-types. Ninetales is an evolved Vulpix, while Arcanine is an evolved Growlithe. Both of them carry Will-O-Wisp, which inflicts a Burn, drastically reducing Attack. If possible, use a special attacker against these two. Or you could hold a Rawst Berry to remove the Burn.
His third Pokémon is Centiskorch, an evolved Sizzlipede. He will Gigantamax it straight away, so you should have a Dynamaxed or Gigantamaxed Pokémon to match it. Rock-types are ideal, because they resist all of its moves, while inflicting quadruple damage. Water (if they're fast enough) and Flying-types should also work.
Gigantamax vs Dynamax
Starting from now, Gym Leaders will Gigantamax their final Pokémon, instead of using Dynamax. Essentially, it's the same thing, but Gigantamaxed Pokémon change their appearance and gain access to a unique G-Max move. However, there's no difference in stats compared to their Dynamaxed form, so no need to be afraid!
You may have received a Meowth, Pikachu and/or Eevee that can Gigantamax. Other Gigantamax Pokémon can be obtained by catching them after a Max Raid Battle. You can tell if you're encountering a Gigantamax Pokémon as soon as you inspect the den, by looking at their silhouette.
Put out Kabu's flames and he'll hand over his Fire Badge. Afterwards, you'll also be given TM38 Will-O-Wisp and a Fire Uniform. Exit the stadium and Hop will take you back towards the entrance to the Wild Area. Before the two of you depart, some surprising guests will come to say farewell. Eventually, you'll regain control in the Wild Area, just outside Motostoke.