This is not a very common knowledge that the GRD contains ND filter. Unfortunately, the truth is that the internal ND filter is used/available only in fully automatic mode (camera logo on the mode dial) and only for apertures between 7.1 and F11. The purpose of ND filter should be to minimize the lens diffraction effect (image distortion caused by too small apertures). According of this dc.watch.impress article
the Ricoh engineers examined a possibility to add (via FW update) an option to turn ON/OFF the ND filter manually. But it seems, they've decided not to release such FW addition.