I've never really liked Stew Smith's articles. I guess they aren't bad, but I find better information elsewhere. I find it hard to believe that 2500-3000 calories is enough to add muscle or fat to someone training to be a Ranger. I was under the impression that their training was pretty intense. Last summer when I was training hard (I wrestle for a D1 college) I ate 4000-5000 calories every day and I lost weight. 1500 would just about kill me.
This winter I was sitting out with a knee injury and I decided to put on some muscle. I started eating all the meat, cheese, eggs, and whole grain foods I could (seriously, I ate till it hurt 4-6 times a day), along with a weight gainer shake. If you're trying to gain weight don't avoid fat. Eat plenty of healthy fats. It's necessary to rebuild muscle and is essential to testosterone production. Cook with olive oil, eat nuts and seeds and fatty fish. There's no trick to gaining weight. A healthy diet that was very high in calories paired with an intense lifting routine got me from 135 lb to 157 lb. in a few months. I also went from 7% body fat to about 11%, but I've eased up on the food and the fat is already disappearing.