Young children are much more likely to communicate their needs and feelings through behavior than through language. One of the biggest challenges facing parents and teachers is learning to understand the language of behavior and responding in a helpful way. Sometimes doing both is hard. Responding appropriately can be especially difficult when we are responsible for the well-being of others and a child's aggressive behavior, such as hitting, pushing, scratching, or biting, hurts or frightens other children. I think the most important steps you can take that you haven't mentioned are solicit this child's feelings and help him find words to explain them, engage him in problem solving with the other children in the argument, and listen with a "third ear."
If weeks and weeks of consistently doing these things doesn't work, try to find out if there are important behind-the-scenes issues going on in Gus's life that cause him to act out so often.
Offer Support and Suggestions
In situations like this, when a certain child tends to strike out, teachers should try to stay near him as much as possible. Remind him of appropriate behavior, using such phrases as "Gus, we don't hurt people. Tell Kandi what you want." (It might be something she's playing with, or something he's playing with and doesn't want to share just yet, and so on.) Or try, "You don't like what Alberto said to you? Tell him. Say, 'I don't like it when you say Fm a stinky stupid.'"
While you're doing all this, try to help him work through the incident so that it ends in a conclusion that is satisfying to both parties.