Two summers ago, I went from 155 to 170 in 7-8 weeks. The training I was doing at the time had an insane amount of volume and some extreme conditoning(I was under a Personal Trainer). The weight that I gained was pure lean muscle mass.
If I can remember correctly I was eating 3-5 meals per day, trying to reach 3000-4000 calories(Multiply your BW x 20, 155 x 20 = 3100cal/daily). I kept the food as clean as I could, eating fried foods on the occasion. Every night before bed I would mix a shake having 600-800 calories. Don’t buy weight gain shakes, they’re a waste of money!
My typical shake was:
8oz whole milk
1 scoop 100% whey
1/2 cup oats
1 banana
2tbsp PB
Occasionally 2tbsp of Canola Oil
The weight gain should come fairly slowly. You should only expect a 1-3lb weight gain per week. Its very possible to gain 30lbs in 5 months. Make sure you stick to the core lifts, you should expect strength gains as your weight increases.
Here are some articles that you should read:
http://www.johnberardi.com/articles/nutrition/masseating_1.htm