Can you use a Legendary in a Gym Battle?

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, it is perfectly possible to do this. I just used an Articuno to attack a gym, myself.

You can also use multiples of the same legendary in a gym battle. The most extreme case is shown in this video:

The trainer uses a team of 6 Articuno to attack a gym.

Yes, you can use Legendary Pokémon in raid and gym battles

Niantic Updates

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.

Yes, you can use legendary Pokémon to attack Gyms.