Great players don't necessarily become great coaches. What is Gasnier going to teach them? His shimmy and acceleration? He could try, but talents and gifts are not transferrable skills.
Any particular "young backline" is going to have its own shortcomings. For example, perhaps players aren't playing for each other. Perhaps the inside man is not playing to take advantage of the skills of the outside man and vice versa. Maybe individual players can be more conscious of the shoulder position of the defender, or not stand too shallow, or learn to read partner signals better, or whatever.
The thing is, there is no good reason to think that Gasnier could recognize, vocalize, and coach such things better than anyone else.
Now if it were technique, like Sowie showing how he punts for distance, sure, it makes sense to bring him in. But Ryles may just be bringing in Gasnier bcs Ryles has a shallow notion of what coaching in fact entails.