Lilith- Ghost type Gym Leader who is actually a Evil Team Leader who prefers Dark Types.
Team Leader Team;
5. Mega Gyarados
6. Female Incineroar (her Ace)
If I were a gym leader I would probably specialize in a Dual Type gym, since it's really kinda a cool idea, and it would include both Flying-type and Water-type Pokemon, since those two types are the ones I have most consistently used in every adventure, even if mostly on account of HM requirements. (That and it still fits the bill of gyms being able to be taken down largely with one type, in this case being Electric.)
My gym would be designed around the idea of traveling from island to island using the wind from different directions, riding on small boats with sails you can open at will and close automatically upon landing. You would have to collect four parts of a medallion from four islands, battling any trainers you collide with along the way as well as the trainers guarding those medallion parts on each of those islands, then come to a central island and insert the medallion into a pedestal, which opens the way to the crescent cove at the North-most point of the gym, which normally has strong winds and waves that prevent direct entry.
The wind is supplied by fans that turn on and off in a pattern, so if you're using the boats you have to time setting sail right or else get knocked off course, often right into another trainer's line of sight. Of course, if you're savvy you can just use your Pokemon with Surf or its equivalent, but you'll still be blown off course if you get caught in the wind as it creates a strong current that drags you in the direction it's blowing. If you are in a boat and stuck in a spot with no wind, such as after being blown off course, you can choose which direction to sail in afterwards, but are locked into that direction as if you were moving on an icy floor.
Trainers in the gym will have different types depending on where you encounter them. Trainers in the boats in the water have Water types, while trainers on the islands have flying types. It's entirely possible, either through strategic surfing or by timing your sailing right, to only face four trainers before fighting the gym leader, but there are nine trainers in the gym not counting its leader.
Waiting at the North-most island is myself. My team would consist of the following Pokemon:
Staraptor (Knows Double Team, Acrobatics, Roost, and Hurricane)
Pelipper (Knows Rain Dance, Protect, Water Pulse, and Brine)
Primarina (Knows Captivate, Dazzling Gleam, Brine, and Rain Dance)
Lapras (Knows Ice Shard, Blizzard, Brine, and Rain Dance)
and finally, my ace, Noivern. (Knows Hurricane, Tail Wind, Air Slash, and Double Team)
Upon defeating me, the intense winds brought about by the fans will die down to a resonable breeze, aside from a single fan ready to push a boat back to the entrance when you're ready to leave. Before you go, I hand you my badge and a TM for Brine, a move used by most of my Pokemon.
My badge would be the Tempest Badge, a badge designed to resemble the winding S shape of a raging tornado with a cloud at the top, with some small little sapphires cut in the teardrop style speckled about the tornado section to symbolize the rain in the storm.
"No matter where our destiny leads,
I'll be there for you,
Always come through,
And that you can believe."