Gonzales didn't actually testify before the committees. To testify means to tell the truth, and he didn't tell them anything. He did send them a very strong message that the committe members have not come close to understanding.
Watching Gonzales speak to the committees, I was struck by his bovine complacency, his low-key, non-confrontational, unruffled, calm and easy demeanor. For a man being questioned about his obvious deceits, disingenuousness, equivocations, dissembling, lies and falsehoods, he appeared to be not the least bit perturbed. He answered questions about others in the Justice Department by saying that he had no knowledge of any of that. His answers to his own involvement were uniformly that he couldn't recall. He avoided answering any questions about anything.
The message Gonzales was sending the committee members that they never picked up on is that he is the prototypical Republican, works only for George Bush and is therefore immune to any questioning about anything. He showed the committees very clearly his impenetrable and irremediable ignorance and arrogance.
His message to the committees was that his Republicanism protects him from being questioned by anyone. He let them know that he has no opinion of them, they are not worthy of wasting an opinion on. They are so insignificant and inconsequential that they cannot raise his ire in the slightest. Gonzales told them by his actions that they are completely irrelevant to him and can be dismissed as of absolutely no importance. He doesn't hold them in contempt or in high regard, he has no feelings at all about them. Once they cease to be unquestioning, loyal Bushies, they cease to exist.
The committee members' outrage at his refusal to answer questions was misplaced. It assumed that he was concealing what he knew to be illegal acts. That's the crux of the matter, Gonzales wasn't hiding anything, he was just treating them to the complete lack of consideration he feels they deserve. To Gonzales, the committees are not less or more than nothing, they are just nothing. He feels that he doesn't owe them anything. They're not worth lying to. They're not worth telling the truth to. His appearance before the committees was calculated to be more insulting than his non-appearance. He came there to make clear his utter disregard and disdain for them. And he did it beautifully, as only the fanatic, true believer, oblivious to any concept of obligation to his fellow man, can.
He couldn't have done it at all if he hadn't appeared. That's why he appeared, in a typical Republican performance, to rub their faces in it.