it all depends on your body type now.. if you're fat, you'll lose weight.. if you're skinny, you'll get more muscle and gain weight.. I gained 20lbs while at basic.. you're gonna be at benning for a long ass time and it's gonna suck.. but be glad you're airborne infantry... hopefully you'll get lucky and get sent to the 82nd..
No, they usually try and fatten the recruits up because body fat is energy reserves. The more you have the more energetic you are.
Some people try to lose weight anyway, but if they find out you'll get in trouble and they'll do that reverse liposucktion on you to make sure you're fat.