I know you cannot place a Legendary Pokemon in a Gym to defend is, and I do know that you can use a Legendary to fight a Raid Boss. But I hear some people saying that you also cannot use a Legendary to attack a Gym. Does anyone know if that is true?
Yes, you can use Legendary Pokémon in raid and gym battles
While Legendary Pokémon will help them take on the toughest Raid Bosses and Gym Battles, they’re not willing to leave their Trainer’s side, so they can’t be left to defend Gyms.