Think of how few major gaffes have come from the podium during Robert Gibbs's tenure. I'd guess that "things that accidentally came from Robert Gibbs's mouth" cost the administration no more than a few day's worth of news cycles during his stint to date. The man not only can hang with the best of them, but he can show them how it's done.
I can really only recall two errors Gibbs made: the "professional left" interview, and the Cheney smack-down from '09 (that makes the list solely because the story became about the White House response not showing enough concern about terrorism, not because he was wrong about Cheney and the GOP). Aside from those two, I can't think of a major misstep, at least not anything on par with the Bush Administration's four flaks. Though perhaps that's due more to the relatively light output of scandals from this Administration than to his talent as a spokesperson.