Interesting responses from everyone. I will add my thoughts since I do consider myself a "hardgainer". The biggest thing I learned is EAT EAT EAT, the more calories the better. I have spent my entire life as that "skinny guy" Until i found a training program that worked for me I was stuck being skinny. I started doing the big 3 lifts, eating the right foods and eating alot. The biggest trick has been like what was said before.
Being able to take in more calories than you are burning.
SKinny guys need to stop looking at the so called "experts" and trying to do what works for a small percentage of lifters. Compound exercises like Squat, DL, Bench, Presses and body weight has put more weight in muscle on me than anything else I tried.
and again EAT EAT EAT.