Realistically it should be as light as you can build it. That said when the GL was designed it was intended to work with full size standard servos and 4 AA batteries in a plastic holder. Modern servos and batteries are much smaller and lighter so the additional weight in covering and paint might just be a wash. The real concern then would be the CG. The GL has quite a large CG range. It will fly with the CG anywhere from the spar back to 45% of the wing chord at the center. Moving all the radio gear as far forward as possible and using very light pushrods will certainly help keep the CG in check. Heavier typically means you need to fly a little faster and use lift more efficiently. It also means the sailplane will deal with wind a little better.