I use resistance trainig to help me in triathalons. When I'm not in race season I am 10-15 pounds heavier and I still do a lot of cardio. You could cut out your cardio if you do cardio to gain weight. I am sure you have heard protein, protein, protein. But if you do not eat enough carbs (which produces insulin) your body canibalizes itself and starts uses your muscles as a resource for energy. Hence you lose all the hard work. My advice is eat imediatley before and after a workout. Eat many medium sized meals a day rather than 3- or 4 huge ones. Your body wastes a lot of that potential fuel when it all goes in at once. I would suggest at least 6 meals a day with atleast 3 protein or meal replacment shakes a day good luck.