Gym Leader Only Pokemon
Genesect
Pheromosa
Melmetal
Deoxys-Def
Deoxys-Speed
Landorus-I
Dracovish
Galarian Darmanitan
Urshifu-Single Strike
Tornadus - Therian
Magearna
Shaymin-Sky
Blaziken (Base Form)
Cinderace
Spectrier
Greninja

Gym Leader Only Abilities
Arena Trap
Battle Bond

Banned From all gyms:
Moves:
Fissure,
Horn Drill
Guillotine
Sheer Cold
Double Team
Minimize
Baton Pass

Pokemon banned from being used in challenging teams:
Aegislash
Magearna
Melmetal
Regieleki

Battle Bond (Ability) is also banned from being used in challenging teams

Banned from particular gyms:
-Bans are subject to change, if you're ever confused don't hesitate to ask here or by messaging a staff member in game
-The rule of banned pokes not allowed for challengers to use versus specific gyms applies only to the challenger, not necessarily the gym itself

Bug:
Charizardite
Hawlucha
Talonflame
Corviknight
Volcarona
Pinsirite
Aerodactyl
Victini

Dark:
Lopunnite
Sirfetch'd
Clefable
Mawilite
Azumarill

Dragon:
Cloyster
Tapu Lele
Slurpuff
Altarianite
Comfey
Glastrier

Electric:
Zygarde-10%
Excadrill
Venusaurite
Garchomp
Chansey
Blissey

Fairy:
Scizor
Metagross
Stakataka
Kartana
Celesteela

Fighting:
Hatterene
Mimikyu
Talonflame
Alakazam
Comfey
Psychic Surge
Hawlucha

Fire:
Drizzle
Sand Stream
Terrakion
Sand Spit
Diancie

Flying:
Weavile
Kyurem
Ninetales-Alola
Cloyster

Ghost:
Blacephalon
Absolite
Mimikyu
Sirfetch'd
Obstagoon
Gengar

Grass:
Volcarona
Talonflame
Alolan Muk
Charizardite
Pinsirite
Hawlucha
Drought
Slowbro-Galar
Slowking-Galar
Emboar
Darmanitan

Ground:
Serperior
Drizzle
Kartana
Grassy Surge
Greninja
Comfey
Venusaur
Kyurem
Cloyster
Azumarill

Ice:
Terrakion
Scizor
Metagross
Lopunnite
Blacephalon
Kartana
Volcarona
Jirachi
Lycanroc-Dusk
Infernape
Cloyster

Normal:
Corviknight
Medicham
Conkeldurr
Hawlucha
Sirfetch'd
Terrakion
Zapdos-Galar

Poison:
Alakazam
Landorus-T
Excadrill
Heatran

Psychic:
Mimikyu
Volcarona
Absolite
Hoopa-Unbound

Rock:
Scizor
Metagross
Gliscor
Zarude
Kartana
Venusaurite
Excadrill
Grassy Surge
Breloom

Steel:
Gliscor
Infernape
Magnet Pull
Breloom
Heatran
Charizardite
Volcarona
Blacephalon

Water:
Kartana
Grassy Surge
Venusaurite

If you have something you want banned from gyms or unbanned from gyms please leave a reply with a valid reason about why they should be banned/unbanned and not because of a reason like "I like x and I want to use it".