I'm gonna suggest that the "lightweight" G/S is something of a conceit.
Take a G/S and an early '88-'90 GS. Give 'em the same tank, starter, and exhaust system.
I'm willing to bet that there's not much more than 5-6kg between them at this point. Consider it the price of better suspension, stronger and tubeless wheels, better rear subframe, more travel, and a longer wheelbase. Oh yeah, more power. But not as cool.
Someone prove me wrong.