Eat a lot of food, preferably from all macro-nutrients. Make sure to get adequate nutrition(include organs or supplement accordingly). Mix that with your training and your good to go.

I would listen to all of Robb wolf's podcasts where John Welbourn was a guest. He talks about this topic on several occasions. Greg also adds some good insight from time to time.

T the end of the day it's an individual thing, but bottom line is eat, lift heavy, and rest. More specifically, get adequate protein for recovery, eat moderately high fat at every meal except post workout, and don't be afraid to pound the carbs post workout. Some say you don't need them after a strength workout but I have found they are very beneficial.

Mark Sisson had an excellent article on this as well. I hope this helps.

www.marksdailyapple.com/gain-weight-build-muscle/

This might sound counter-productive, but I found doing a modified Stronglifts 5x5 program put more muscle on me than a conventional BB workout (many exercises & reps). I have not calculated the exact amount, but I can tell I have gained muscle. I eat a moderate carb Paleo diet (75-100 grams) with a few corn tortilla cheats (I live in Texas). A lot less carbs than I used to eat. I am 38 and have lifted off and on since I was 13. My gains could be from the lower amount of over-all work or from the cleaner diet, or both. Good luck. Experiment with a few things and you will find out what works.