If someone lies to a GJ, it's a terrible perversion of justice. Period.
But for it to be a crime, the lie has to be on a "material fact."
Is how Libby learned of Plame's status a "material fact" even if revelation of her status, by anyone, at the time it was revealed, did not violate any laws?
Yes, to the extent he lies about how he learned of Plame's status. Part of the inquiry was "how did Plame's status make it's way to the press?" Libby was asked "How did you learn of Plame?" and according to the indictment, his answer was in the form of "I learned it first from the press." But in fact, the prosecutor has evidence that Libby first learned of it from his own inquiries to the CIA. If he'd told the GJ that he first learned of her ID from the CIA, then played mind games with the press, probably "no bill."
The "who did the disclosing" line of questioning is a material inquiry to probing a violation of 50 USC 421 - even if other elements of the crime (e.g., "covert" status) are known to be not met.