Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Reference
GV_HAIR_OBJECTParameter: Mode
GV_HAIR_OBJECT_MODEParameter: Count
GV_HAIR_OUTPUT_GUIDE_COUNTParameter: Segments
GV_HAIR_OUTPUT_GUIDE_SEGMENTS