I suggest a weightgainer. I was in the Marine Corps infantry so I know where you are coming from. Unfortunately, between going to the bush and all that zero-dark-thirty cardio, you'll be very hardpressed to gain anything significant. So, with all that calorie deficit going on, you need a protein drink with a significant amount of carbs/cals. I suggest something cheap like N-Large-2. You can get a big ten pound bucket for $40 or something. In order to grow, you need a calorie surpluss...And you aren't going to get it eating at the chow hall three times a day.
BTW, it sounds like you just got out of Ranger school, correct? If so, I take it your active duty? If that's the case, you are pretty screwed if you want to put on a lot of weight. At the most, I was at 190 lbs at sub 8%. Being in the infantry, that's about as good as it gets..