Let’s see... I believe I’ll do a different one for both IC and OOC.
OOC me I’d say the gym would be a Dark type Gym, filled with ninja kids or hex maniacs, using the appropriate ghost, Dark, or bug types. The gym would be designed like a dojo-themed maze, it’s main Gimmick being fake walls, ones that appear real but can be walked straight through. The first one would be introduced by a trainer ambushing the player from within it, with most of the trainers afterwards giving hints as to which wall is fake rather than walking right out and showing you (though I imagine ambushes from it would still happen occasionally). As for my own team:
Alolan Muk, both poison and dark, would also possess the Iron Ball Fling combo to surprise an early mon
Liepard, this one would be focused on annoyance with moves like fake out and sucker punch
Zoroark (ace), comes in disguised as the next mon I’ll mention, Zoroark likes to start with Nasty Plot before moving on in an attempt to sweep with Night Daze and flamethrower
Spiritomb: standard tanky dark type, major reason for existing is to give Zoroark a good disguise, though its curse can prove annoying if you’re not expecting it.
I would be an upper mid-game gym leader, likely the 5th or 6th, if 6th it would mostly be due to the complicated gimmick
I actually have 3 ideas for the IC
First, as the second or third gym leader, Sayaka would lead a music-themed normal type gym, where you find sheets of music and bring them to say, a piano before playing it to progress. This early in the game Gym leaders typically only have three mons, and so we get
Clefairy: a fairly durable mon, Clefairy likes to use Metronome and pray for the best, or wish to heal itself
Audino: like most audinos, all it can really do on it’s own is something like double edge, or considering how early this is, take-down. I do believe Audino can learn recover or wish, any healing would do
Chansey (ace). Chansey would be known for its use of the defense-curl rollout combo, in addition to having take-down, its use of soft boiled to heal would turn Chansey into a simple damage test, in other words can you hurt it fast enough
I’ll tell the other two when I get on lunch break, enjoy this for now