Been there, done that. You will lose weight in boot camp. Period. It's not what you eat, it's portion control. In other words, with 5 min to eat, no matter what they give you to eat, you won't have time to eat it all. After the required 1 full glass of water that the DI's will no doubt make you drink since it's summer, you need to pick and choose what you eat off your plate before time's up. I'd go for the most calorie dense foods available. Drink the mashed potatoes+gravy. Yes, that's right, drink it. Then drink some PB, and whatever protein you get.
After boot camp, you'll go to MCT or SOI, depending on if you're infantry or non-infantry. You don't get much time to eat there either, since it's either 2 wks or 6wks in the field eating MRE's. While MRE's are about 1200-1500 Cal each, and you get 3 per day, you'll get tired of the meals. You won't get time to hit the gym anyways.
After all that, if you're non-Infantry, you'll go to MOS school. THIS is where you'll have a ton of time to train and eat as much as you want at the DFAC buffet. If you're infantry, you'll ahve even more time on active duty. Since if you're not out in the field training, you're sitting in the barracks doing nothing, playing video games.
In short, you'll have no problem gaining back the weight once you complete boot camp and MCT/SOI. Welcome to the Corps!