I want a gym that is entirely done in beige and warm greys, with a slight buzzing from AC and florescent lights.

All my Pokemon will be normal type

Magikarp.

A gym filled with thousands of gym trainers, before you reach the gym leader. Each with 6 high level magikarps with as much hp up and defence supplements that they can take.

The sheer scale of the amount of Magikarps you need to face will make it an immense undertaking.

At the end you get a TM for Splash.

Under water gym. No, not water like Misty. Under water.

I haven’t figured out all the logistics. How will people breathe?

Seaweed mazes and cool cave grottos, sunlight filtering in from above,

There’s rocks and plants and corals and shit under water. My trainers will use all those Pokemon. Not sure about ice yet. That can be in the sequel game – the icy under water gym.

bed

All food based Pokémon. I think that was a gym though? Lady with a cafe though, had a fidough maybe? Me and my Pokémon would be just trying to have a garden, give out food to travelers. We’d all be objecting to being a gym leader lol.

No type theming. The common thread is turtles. Squirtle line, Turtwig line, Shuckle, Torkoal, and I’m pretty sure there are more since Emerald that I don’t know the names of.

Cork board and lots of string. My gym gimmick would be sane type composition, which was not much of a thing in the three gens plus change I played. I’m writing this as a casual who, when I replay this, still use 99.9% straight damaging STAB moves and just don’t intentionally hamstring myself with the picks. Pokemon battling is apparently very deep, but most of it still feels like doggy paddling. If you want to do “clever” things you need to go out of your way to not wipe the enemy out too quickly, and is it really clever then?

Besides, Pokemon writing/characterization/whatever being what it is, e.g. “super nerds” use poison for some reason, instead of using the purported super nerdery to, y’know, pick a team with more than 2-3 types total in it. People in-game even mention type composition all the time, because exposition, and then even the Bestest Guys in all 3 countries mostly don’t give a shit.

So without putting together any actual teams or anything, the idea would be essentially minmaxing based on type strength and weakness. Possibly my player-POV “however many types I can slug with a STAB” or the inverse “however many types I can guard against”. It doesn’t need to be super strong in an objective sense but would make sense late-ish in the game.

My favourite type is fairy so it would be something pink and comfy. You come in and rest and in the end don’t even want to leave. Maybe some kind of café

Giant terrarium. My pokemon would be reptilian analogues like Ivysaur, Feraligatr, Sandaconda, Serperior, Salazzle, etc…

Literally the same exact thing you said, but with actual ghost types wearing low effort ghost costumes.

We’re watching the whole original series as a family and we’re partway through orange islands, ghosts have appeared 3 time so far. Maidens peak has ghastly straight up trolling horny men during an annual festival. Second has all 3 living their best lives in a haunted tower and dicking around with people that try to enter. 2.5 is a continuation of that and I don’t count it as a stand alone, but haunter goes with ash to become the personal jester for a Sabrina. 3 has ghastly and haunter raising a ghost ship to return their dead master’s property, despite being ghosts on a ghost ship, they’re running around with bedsheets on pretending to be ghosts. What do they do when togepi gets lost on the ship? Take it to their comfortable cabin and play baby games with it because ghosts aren’t here to scare you, they’re here to have fun pretending to be ghosts.

Carnival style haunted house, but all of the really dumb pop up ghosts and animatronics are actually my ghost Pokemon lulling you into a false sense of security before scaring the shit out of you. If they think you got what it takes, your cart diverts down an underground rollercoaster to my secret crypt style arena under the spook-house. After the battle, the cart looks like you’re going to take another roller coaster out, but it diverts at the last minute, take a low 30ft roll and ends at a gift shop where ghastly sells you “I survived the spook house and all I got was this shirt” t-shirts and you take a short elevator ride to the exit which is literally right next to the entrance.

Oh, you mean layout and decoration, and not music?..
You Will Be Reincarnated As An Imperial Attack Space Turtle

Space I guess?

I like water types, and there are a few dual type mons that bring in some type advantages to counter what one would typically bring to a water-type gym (e.g. Palpitoad/Seismitoad and Marshtomp/Swampert are all water/ground to nullify electric types, or the Lotad line to avoid grass-type weaknesses).

As for a gym theme, I like the idea of a spa/thermal baths place that functions as the town’s Pokémon Center, with a closed off area for gym challengers. Gym leaders are often important members of the community, so I think it makes sense that the gym leader would have other duties to their town in addition to battling.

I was thinking of what type would fit me… I guess pokemon that will rage over the slightest things.

I always wanted to have a full team of glitch Pokemon:

Ash is wading along the beach on cinnabar island when suddenly he falls over onto the hard ground. Bright light comes from all around as he looks up in shock to see a strange emtpy wireframe space.

As he gets up, a piercing noise shatters the silence, it seems to come from inside his own head. A shadowy, pulsating form fades and crackles into view. An ‘arm’ extends and throws a ball, completely black like a void. Exploding from within is a pokemon…? No It can’t be! Ash stares wild eyed at the strange rectangular form, a chunk missing in its top right corner. It shrieks as it ripples with strange patches of incomprehensible randomness.

Ash’s vision flashes for a moment back to the beach, he’s staring up at his friends as he lies on the sand. They’re saying something but he can’t hear it. He can’t speak, his eyes are portholes and he’s trapped inside. Then he’s gone, back to the broken liminal space.

The pokemon is approaching. Or is it? It doesn’t seem to exist properly, a 2D form in a 3D space that only seems to grow in his vision. Ash looks down to see pikachu by his side, hair standing on end. They exchange a glance and he knows what to do.

The fight is a blur, each new opponent more incomprehensible and impossible than the last. As the final entity is destroyed. Ash falls to his knees, his mind shattered by the infinite cosmic chaos. The strange ‘gym leader’ begins to fade and the world starts to crumble to blackness. A bright light is shining. A voice emanates from all around. “Wake up ash you’ll be late for school.”

Ash opens his eyes and looks around, he is in bed. The world seems so much more detailed, so much more real. A handheld gaming device rests on his chest, white light flashes from its LCD screen as a pixelated title cutscene plays. A single word appears. PoKéMoN™

Is sloth a theme?