Provided you are doing some resistance training then you can gain weight without gaining fat by eating lots of protein, such as eggs, chicken, turkey, steak or lamb - grass fed is best. Turkey leg joints are very good value if you can find them, lots of protein for the money. It is important to eat more calories than you burn though otherwise it wont work, you must exercise and also drink enough water too. Muscle is much denser than fat, but is mostly water so being hydrated is very important.