I'm not sure yet. I've always thought of myself as a hardgainer, but it's possible that I've never trained and eaten hard enough to really know.
I can say that over an 8 month period I slowly lost weight while eating ~ 75 primal grams of carbs day and working out modestly (pushups and pullups). I started out very thin and lean to begin with, and one of my goals was to put on a little weight, hopefully muscle.
Because of this, I started incorporating white rice into my diet almost daily, eating more sweet potatoes, and adding in whole milk and yogurt on days when I need more calories. I also ramped up my workouts starting at the beginning of this year. I followed PBF for ~ 6 weeks, and have been doing SimpleFit for the past few weeks.
Over the last 2.5 months my weight has gone up about 3 pounds. I can't say how much of that is due to fat, muscle, or water. Upper body and quads appear more muscular, 6-pack looks the same, etc., so I'm guessing it's mostly muscle and water gain. I'm trying hard to eat at least slightly above my maintenance caloric requirement, but I don't typically get more than about 0.75g protein per pound of LBM. 0.5g is more typical.
If I continue to gain about a pound or so per month for a year, and I don't appear to be getting any fatter, then I'll probably have to exclude myself from the hardgainer club.