The Great Gatsby, walking in his alert way

I'd like to know if "alert" in this excerpt from the last chapter of The Great Gatsby means "vigilant" or "to walk briskly", "at a brisk pace":

One afternoon late in October I saw Tom Buchanan. He was walking ahead of me along Fifth Avenue in his alert, aggressive way, his hands out a little from his body as if to fight off interference, his head moving sharply here and there, adapting itself to his restless eyes.

Tha,k you.