This is the misconception that is evident in regards to Gase or other QB whisperers. How you do as a head coach usually has no bearing on how you do as an assistant and vice versa. For instance Wade Phillips is an outstanding defensive coordinator but every head coach job he's had has a so-so defense. Joe Philbin is an offensive line coach. He was not coaching the offensive line as a head coach (I'm not sure what he did as a head coach lol)
Some guys were meant to be head coaches, other assistants or coordinators.
We will not know what Gase will be until after the season.