Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Global Position
GV_NEARESTPOINT_INPUT_GLOBALPOSITIONParameter: Point Index
GV_NEARESTPOINT_OUTPUT_POINTINDEXParameter: Point
GV_NEARESTPOINT_OUTPUT_POINTParameter: Distance
GV_NEARESTPOINT_OUTPUT_DISTANCEParameter: Difference Vector
GV_NEARESTPOINT_OUTPUT_DIFVECTOR