I have to agree with Steve that it means the peak height of the gable end. It is just a GUESS on my part.
If they meant to to say the midpoint between the decking (or wall height) and the peak then the zoning rule wouldn't make sense. For example, if the wall height is 8 ft, then the midpoint is 12.5 ft and the allowable peak would be 17 ft (8 ft + 2 X 4.5 ft). If the wall height is 6 ft, which is too low for a garage, the allowable peak would be 19 ft (6 ft + 2 X 6.5 ft). This would not make much sense, although I'll be the first to say that zoning rules don't all make sense!