The gliding distance is unaffected by weight if you maintain the angle of attack for the best lift/drag ration. A heavy aircraft will require a higher airspeed to maintain the best lift/drag ratio than a light aircraft.
Competition gliders carry water ballast, not to fly further, but to fly faster - and they can dump that weight at low level in order to take advantage of the higher speed, for a zoom climb ability.
Pilots that use constant speed schedules (generally much higher than best lift/drag speed) for descent will notice an increase in the range for descent when heavy, simply because they are flying closer to the best lift/drag angle of attack when they are heavy compared to when they are light.