Maybe there would be a 1st Amendment issue if someone wanted to hold a political rally in a state park, but that doesnt seem to apply here.
It’s public property, so in effect it’s owned “by the People”.
So Newsom needs to show a valid reason why people can’t be there. So far he hasn’t done that.
When you’re outdoors in a park, you’re automatically “social distancing” already, so how is it illegal?