There is no formal division between light and heavy equipment in gymnastics. All equipment used in the sport is fairly heavy, as it has to support the body weight of athletes. The only light equipment is used in rhythmic gymnastics.
Competitive gymnastics can roughly be divided into six different categories -- men's and women's artistic gymnastics, rhythmic gymnastics, the trampoline, power tumbling and team acrobatic gymnastics. If someone refers to "light" gymnastics equipment, it is possible he is referring to the equipment used in rhythmic gymnastics -- hoops, balls, ribbons, clubs and rope. These are used to enhance the standing routine of a gymnast.