I myself am sort of a "hardgainer". I lose weight on two or three days of reduced calories eating.
However it is not so difficult to gain. I remember 5 years ago, when I was running marathons and switched to weight training. For months it is was impossible for me to raise above 80-82 kg (180 lbs), until I decided to take nutrition as another training. I began to eat far beyond satiety, drink milk instead of water and go eat something every 2-3 hours. I soon broke the barrier and raised my weight.