Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Center
MDATA_MGGRIDTOOL_CENTER
Parameter: Size
MDATA_MGGRIDTOOL_RADIUS
Parameter: Clones Count
MDATA_MGGRIDTOOL_COUNT
Parameter: Generate
MDATA_MGGRIDTOOL_MODE