Gibbs was an offensive genius and his teams will never get the credit due to the strike seasons in which they won two Super Bowls. Given a choice between Gibbs and Walsh, I'll take Gibbs any day. Ron Wolf is. The Packers won one Super Bowl under him. The Pack did improve significantly, but that isn't the mark of greatness.
Greatness is more than just getting better.
He left San Diego completely void of talent except for a couple of players. I think he did as much or more to ruin his reputation as Jimmy Johnson did in Miami. Twenty years takes you back to 1981, where, even if you overlook what he did for football prior to 1981, he still had a great football mind. It's no coincidence that since he died in 1991 Bengal football teams have not made it to the playoffs. While he was still alive, they went to two Super Bowls and the playoffs four times.