If you point a gun at someone and they're not trying to harm you or threatening you in any way shape or form then yes it's illegal to just point and brandish a gun at someone or people.
If someone suddenly points a gun at you and you haven't given then a good reason for them too then yes it's illegal for them to do so and they can go to jail.
Pointing a gun at someone and brandishing a gun when not in self defense is a crime in most if not all states.
If someone is brandishing and pointing a gun at you then you should call the police and they'll most likely be arrested for it.