Alright my memory might be off but did Jim say somewhere that Gatekeeper avoids using too much magic or something like that? Then that leads to this quote by Jim:
Indeed. In the Dresden Files universe, magic is the essence of creation itself. Constant exposure to it through use changes the person who uses it in a number of ways, not all of them as obvious as physical recovery and longevity. The more exposure, the more dramatic the changes.
So if my memory was accurate, could Gatekeeper be avoiding using it because of the changes it's creating in him? If you use enough magic do you become something more? A lower level god, or something like that? Wizards get more set in their ways with time (I think Jim said). So if you use enough magic, over enough time could this basically cause you to become your magical beliefs or a Mantle bearer? Almost like you use so much magic over a period of time you come to the point of no return where you elevate to a stronger Being but more limited in Choice because you are locked in to your ways?