IMO the GSD diet is okay for short-term only. The calories are very low, unless he’s getting all of his metabolic stimulation from resistance training. In other words, GSD+lots of cardio= muscle loss, again IMO only.
I suppose my own diet now would be similar to GSD, with slightly higher calories closer to the 13x number, lower carbs with a very moderate carb-up every 5-7 days as needed.
The leaner I get, the lower I take the calories and carbs and the more frequent I have to utilize the carb-ups.
Oh, and if he can stand the stuff, HOT-ROX works like mad.