Since this is going to be a Fighting gym, what better way to make a Gym to its theme than a gauntlet?
In short, if you want Gustav to take you seriously in a Gym battle, you need to prove that you're both a competent battler by beating his Gym Trainers, as well as physically able to survive his gauntlet at the same time with your powerful manly-man muscles you never knew you had. (Applies to women too! Mental imagery incoming.)
There will be three parts to the gauntlet:
FIRST PART - SPEED: The first room is a flat room, looking similar to a dojo with the paper screen doors as walls, and the bamboo mat floors. Here, Trainers will shoot out of any point on the walls of the room, and proceed to shoot towards you to attack. Hit the B button to guard their first attack upon contact. At this point, a quick-time minigame will play. Imagine the waiting dialog box for fishing [..... Something is on the hook!], except WAAAY more dynamic and fast paced. Whenever the enemy attacks, you hit B to defend. When the enemy backs off, you hit A to attack. You need to do this quickly though, or the attacking Trainer will punch you and you'll go flying, tearing a hole in the paper screen walls and having to re-enter the room. If you successfully fend off a Trainer, you knock them away just long enough to grab a Pokeball, and the Trainer battle begins. Beat 3 Trainers in a row to advance to the next challenge. If you beat one Trainer in a battle, but get knocked out of the room by another, you start from your latest win, so no resetting back to the first trainer to grind. The number of attacks/defenses per Trainer is something I'm not sure about; personally I want it to be a harder number each time you beat a Trainer, so it would scale as 3/5/7 attacks. Either that, or it would be a flat rate of 5. Hopefully you're fast enough!
SECOND PART - BALANCE: This next room doesn't have a floor. Instead, the 'floor' is a large array of hardened bamboo stalks. You need to traverse your way across the Bamboo Stalk Gauntlet and defeat all the trainers in there (most likely just 2 or 3 of them). To move, face in the direction you want to jump and hit A, and you'll jump it like a ledge (in-game, this probably wouldn't act like a ledge, but rather a thin walkway with the 'jump' move animation). Each time you move twice, every trainer moves once. You may not battle a Trainer from the front. If you try, they'll kick you off the bamboo stalks and you start at the beginning of that room. Instead, you must move around and sneak up behind the trainer, then initiate a battle. Beat all 2 / 3 Trainers, and the Balance Challenge is complete!
THIRD PART - POWER:
If we're going with a lava theme, the puzzle could involve having a series of rocks that are blocking your path while simultaneously holding back flowing lava. If you break the wrong rocks, lava ends up completely blocking your path, forcing you to leave the room to reset. Breaking the correct rocks in the correct order lets you proceed.
Basically this, but I'll try my best to give it more detail. This will work like a maze flooding puzzle like Cianwood Gym from HGSS (I think). Break rocks with your bare, manly fists (or just push them, you sissy) to either clear paths to the rest of the maze, or to lava flows. Some lava flows will block off the map and force you to exit and re-enter the room. Some lava flows will lift large stones by filling in pits with lava, opening up new paths. Trainer battles are optional here, and can be avoided.
But are you a little man-child from a little man-child country, or are you a MANLY-MAN?! Fight those Trainers, you weaklings!
Traverse the maze and complete it to get to the GYM LEADER'S FINAL ARENA.
FINAL PART - GYM LEADER: The final room is the same pit of lava you saw Gustav swimming in...at first. Gustav himself is doing a front crawl in the lava, all the while talking to you because he's manly enough to multitask:
"Oy gevalt, yeh made it, didn't ye, lad?! Ach, I knew y'had it in yeh! Now that yeh've proven your brains AND your brawns, yeh'll get a right cataclysm from me! Prepare yerself, lad!"
At that exclamation, a giant steel cage shoots up, surrounding the lava pit. A stone large enough to stand on rises from the lava, and you jump onto it for footing. Now, Gustav makes his entrance. He dives headfirst into the lava, and disappears for a few seconds. You hear and feel a low rumbling. As it intensifies, Gustav rises to the surface, riding on a
geyser of lava ten feet tall, made by the sudden appearance of Heatran using Magma Storm. Turns out, the man is on friendly terms with a primordial magma beast.
Your trainer briefly considers dropping all pretense and worshipping this god among men, this shining example of manliness.
And the battle begins.
(I don't mind if only one or two parts of this gets used, I just had to get this idea in writing.)