How good of a College Head Coach would Gruden be?
I don't think he'd be all that special.
He has a lot of notoriety, charisma and a Super Bowl ring, so he would do very well with recruiting kids. If he can learn how to sell academics on top of his NFL experience with football, he'd do well with recruiting parents.
But as for coaching college kids? He's too complicated. One of his big issues in the NFL was player development and running a system that was too complex for most quarterbacks.