<<The fact is, he pled guilty in exchange for community service and probation.>>
OK. So he himself ADMITTED that he did it, in the first instance.
<<After he pled guilty, the judge accepted the plea deal>>
Uhm, aren't you leaving something out? Before the judge can accept the plea deal, he has to accept the plea of guilty. Has to satisfy himself that it's not coerced. Hears all the facts of the case read into the record by the prosecutor. Asks the accused if the facts as read are substantially true. Hears the accused admit the facts read by the prosecutor are true. THEN, he accepts the plea. And after that, THEN he approved the plea bargain.
So at that point we know with 100% certainty that this piece of shit viciously assaulted a gay man in a bar with no provocation and terrorized, threatened and humiliated him for about half an hour without letup. Nice.
But that was not the end of the story. The little shit had apparently snuck the plea deal past the court by pretending that he was never in any other trouble with the law, when in fact he was under indictment in the rape case. Why should the judge let the plea bargain stand when it was obtained by deceit and deception? If he let this one stand, other accused would be encouraged to lie to the court about the extent of their involvement in the criminal justice system in order to secure a more favourable plea bargain.
So what happened next? As you yourself admit: <<. . . the judge heard about the indictment for the rape case (which never happened) and the judge revoked the deal.>>
Naturally, not knowing at the time what the outcome of the rape charges would be, the judge was probably outraged by the deception that had been practiced upon him and revoked the plea bargain, putting the punk on trial on the assault charge. And guess what? After a full and fair trial, [in the course of which his star defence witness lied his ass off], after a full defence, and after being given every reasonable benefit of doubt, HE WAS DULY AND PROPERLY CONVICTED OF THE OFFENCE. What a surprise.
<<How do you appeal a guilty plea? Especially a plea that was predicated on a deal the judge initially accepted and only revoked based on a lie?>>
You just answered your own question. When the guilty plea was revoked by the judge, there was no plea to appeal. A trial and conviction followed. You SHOULD have asked, how do you appeal a conviction? It's easy: you have to start with valid grounds for an appeal. Of which, apparently, there were none.