He is a pro coach now having spent 11 of the last 12 years in the pros. Pro head coaches don't convert to college head coaches at a high level of success and vice versa.
If you are under the personal delusion he would leave GB for ND, he'll have at least 3 years in GB if he is anything more than mediocre,. If he is worse than that, he isn't a head coaching candidate. If he is better than that, he will be in GB for a long time.