Judge Corbett's gavel might technically belong to him, but since he largely represents the justice system as a whole in this book—he's the only judge we meet, after all, in a book that features a pretty major trial—we can think of his gavel as representing the justice system, too. After all, it's the tool judges use to maintain control over the court:
He lifted the heavy mahogany gavel. I was surprised to see him using the gavel I had sent him on his sixtieth birthday, since I had never received a thank-you note. He brought the gavel down with a thunderous bang. (97.22)
That the gavel is representative of the justice system is made clear by the fact that Judge Corbett's connection to it isn't personal—he doesn't use it for sentimental reasons, as evidenced by the fact that he didn't even bother sending Ben a note when he gave it to him. And check out the use of "thunderous"—the justice system is as powerful as a storm, and the participants in the trial are helpless in the face of such might. They ultimately have limited sway over the ruling, and instead are at the whim of Judge Corbett and the justice system in general.
And in this case, that's one corrupt justice system to be at the mercy of.