I saw a documentary about him getting fired at Oakland, hired by Tampa Bay, and facing his old team in the next Super Bowl. The Raiders did not change anything in their playbook that Gruden himself drew up.
Now, I think Gruden knows his football, but he won that Super Bowl in large part due to Oakland's stupidity. If you know the strengths of your own plays, you are going to know the weaknesses and how to defend them.
I will be shocked if he takes another coaching job. Like John Madden, he is locked into color commentary broadcasting. If he coaches again and screws up, he would have a hard time regaining that job. Oh he talks getting back, but that is just his own self-promotion.
The idea of having him is good, but that is probably all it ever will be.