My 2005 GS had no final drive drain plug or fill plug.
There was only a plug at the rear of the casing, mid-height (9 o'clock looking from rhs of bike.
To drain the oil, you took out this plug, then released the torque arm and rotated the drive 90 degrees anticlockwise, pivoting on the paralever bolts. You removed the ABS sensor before rotating, and this hole allowed air to enter - no breather in the old one either.
After draining, you rotated the drive 90 degrees clockwise, meshing the driveshaft splines etc. Refil 180ml GL5 through the sensor hole using syringe..