Do you avoid High Fructose Corn Syrup?

Don't consume lots of fructose to gain weight! Lots of fructose is bad news in terms of your liver. (yay metabolic biochem class!) That's a terrible idea. If you really do want to gain weight, exercise your muscles and eat lots of protein. That way you'd put on weight in a healthy and more attractive way without straining your liver to do it.
Why do you want to gain weight, anyhow? Unless you look like skeletor, I don't see the point in it. You aren't underweight enough to look like you're starving. Your "BMI" is just below what is considered to be the lower limit of healthy, but only very slightly. That "BMI", unless you went to a doctor and had a whole test performed, doesn't mean very much. It doesn't take into account bone density, bone structure...not all peoples' frames are the same at all. Also, its been shown from many angles (cancer, heart disease, metabolism-related oxidation (ie aging), etc) that animals who consume less calories live longer and develop far less health problems. Stop trying to gain weight and accept your body. :)