That sounds impressive!
P90X sequel + GMAT - certainly your body will be competing for resources. Your body will need carbs going into the workouts and you'll need protein to replenish immediately after, followed by casein protein (cottage cheese) right before you go to bed.
The P90X exercises are tough. Be careful you don't overly exert yourself - some of those sessions go on for 40min+. Be aware that this length of time will tire you out so drink lots of water and food.
High intensity interval training is one area that P90X doesn't focus on as much - it's also another approach to burning more fat in as little time as much as possible (same concept as GMATPill)- the reason is because your body spends the 3 hours after workout bringing your body back into homeostasis and bringing your depleted glycogen levels back to normal. This process burns calories at the rate of slow jog for 3 hours - while you just sit there and breathe oxygen and let ATP do the work.
Hopefully you'll see some nutrition passages in your GMAT studies! I recommend efficient studying so discover shortcuts for yourself and aim for the stars!