I know this has probably been beat to death in the past but I would consider myself a former hardgainer (125lbs at 18yo), and I have discovered the missing factor for myself was quality sleep. I naturally filled out after 18 to about 160, and H-drol put me at an unsustainable 190, but I am all-natural now and a solid 180. Steroids have been proven to make permanent physiological changes(more motor neurons), but I still dropped back to around 165-170 eventually after cycling.
Anyway, long story short is that I had undiagnosed TMJ that was wrecking my sleep. Last time I visited the dentist he discovered unusual wear in my back molars and could feel grinding in the jaw joint. He said surgery would probably leave me worse off than I started, and recommended a do-it myself mouth guard(awesome dentist) for 20 bucks. I'm well rested after 6 hours where I used to sleep 10-12 when I could and still felt like crap.
tl;dr:
I wouldn't look to nutrition first if you are a 'hard-gainer', sleep is when GH and test levels are at their highest; if you aren't sleeping right eating enough to gain weight will probably just make you fat.