I don't live in the US, so I'm not familiar with the law there, but I don't understand why a peacefull gathering would be illegal ? Are large gathering illegal somehow ?

“Peaceful” gatherings are not illegal as long as they follow the rules.

https://www.aclu.org/know-your-rights/protesters-rights/

In every country in the world the rules of protests are legislated.

Peaceful protests can turn violent or break the rules and then police can shut them down.