There isn't an organization called "Antifa" anyway. It's an ideology. The general concept is just not liking fascism, which nobody should. The problem is specific individuals organizing and recruiting for particular acts of terror, like Eric Clanton. He is quite evidently a terrorist in methods as well as ideology.
So you couldn't really just declare "antifa" to be terrorist any more than you could declare "white supremacy" terrorist. You could, however, declare the Aryan Brotherhood a gang. Or even some unnamed conspiracy between, say, Eric Clanton and Does 1-99.
This doesn't have the legal impact some people may think it does. It doesn't just magically give you probable cause for search warrants or let you arrest anyone who expresses anti-fascist sentiments, white supremacist sentiments, or whatever.
However, it can state a law enforcement priority and that certain types of crime or other illegal activity are to be prioritized for investigation and enforcement.
In this case, antifa has been allowed to become an absolute menace to innocent bystanders and the public at large because of deliberate law enforcement inactivity, and this needs to stop. Enough is enough.