Being a hardgainer myself, the most effective method to gaining weight was running / high intensity cardio.
I went from 115 pounds to 165-170ish over 6 years, the biggest gain I've seen was when I added long distance running to my routine. It allowed me to eat large meals consistenly while staying lean. I was always told to take it easy on cardio because you burn calories that are needed for a hard gainer... surprise surprise, my best bulking cycle was from 150-170 pounds while running 30kms + a month. Your metabolism speeds up, so does your conditioning. Try it!