I actually made a template like this. I think it's a fun idea to expand on more in the future. To fight Gordon, you have to get through a haunted inn maze with trivia/riddles.

Blake's Gym has piano tiles, the challenger will be given some clues as to which tiles they have to press in order to advance, if they do the wrong one...well, nothing bad will happen ksgsgshs they just have to start again. There's 3 sections with each section having a longer melody to play and after successfully playing all three the stage will open and Blake will pop up and start playing a song. Fun fact, the gym starts up silent, the melodies the challenger "discovers" end up becoming the gym music!

If Mary had a gym she'd have a kind of test of courage to get to her. You need to walk through a kind of mini haunted house to get to her.

Icebella's Challenge pushes the trainer to plan ahead by making them select a set of Pokemon to use, one for each of the hot spots across the city they have to find. At each spot, they'll fight one of her Trainers with that selected pokemon-only. It acts as both a challenge and as a way to promote different cultural hot spots.

For Norra's, trainers have to find her in Karnelt Town and she'll ask them to lead her back to the Gym.

Which, if they get the location correct, will present them with this cozy 2 story house out of the way and between the Town and it's Castle. Nothing really standing out that makes it scream "This is the Pokémon Gym" to people, until people take a look behind the house. There trainers are met with it's grassy plain Battlefield, a few chairs, a couple of tables and logs for additional seating fenced off from the dense forest area a little further out.

Phillip's gym is a racetrack. In game you'd have to use the bike and move very fast/precisely if you want to avoid the trainers.

Polly's gym is a lot like Roxie's, so a concert stage. You'd have to navigate past some rotom-possessed-speakers that'll blare out periodically, pushing you back(usually into the way of a gym trainer).

If Estella did become a gym leader, she'd choose Fire types with a team somewhat focused on Sun support.

I'm not quite creative enough to come up with a gym puzzle on the fly alas, but she would definitely have something themed either around the fire type itself or fashion/modeling.

In an AU where Briar accepts becoming Kabu's replacement as the Fire-type Gym Leader, I feel like she would re decorate her gym to be more royal-esque, as for the puzzle, I was thinking about something like hide-and-seek with Nickit LOL Like you have to find some object Nickit has (probably a crown) to win the challenge, but she keeps running around, hiding, and she will make you stumble across the Gym Trainers to battle until you eventually catch her LOL And then you get to battle Briar :3c

Tyler actually did get the chance of being the Flying Type Gym Leader, but he turned it down, because there's already a Flying Gym in Johto, and he wants to become a Champion instead. Though if Falkner steps down, Tyler would probably keep the same puzzle Falkner has because laziness, but if the trainers decide to wuss out and escape the gym trainers, he'll blast on some giant fans to try to push them into the trainers as a big middle finger XD. Its possible to push through the air, but its his punishment for trying to take the easy way out

Balei was a trial captain in her youth and her challenge was to show her 5 dance moves (I.e. Dragon Dance or Feather Dance) and the trainer would have to use them in front of certain statues that would release a wild Pokémon of the type of dance move you used to battle!
The Totem Pokémon would be A Kommo-o who summons an Oricorio, Volcarona, and Haxorus who all know dance moves.
This would be in Poni Island but you’d meet her in Malie City in order to receive her trial!

Jasmine: A forest that has little clues to where she is, taking the wrong path will end up in another member of the gym, when the challengers find Jasmine She is in a forest glade with a tiny lake. (The Gym is actually a test in order to see if the challenger is able to explore the mirage forest near the town and not get lost)

Ace (A guy I'm still workshopping): His Gym is actually a Casion where you can win chips by pokemon battles or by classic gambling then you have to buy the challenge letter with the chips of the casino