Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Falloff Matrix
GV_MG_FALLOFF_MATRIX
Parameter: Sample Position
GV_MG_FALLOFF_POSITION
Parameter: Normal Direction
GV_MG_FALLOFF_NORMAL
Parameter: UVW Coordinate
GV_MG_FALLOFF_UVW
Parameter: Array Size
GV_MG_FALLOFF_ITERATIONCOUNT
Parameter: Index
GV_MG_FALLOFF_ITERATION
Parameter: Value
GV_MG_FALLOFF_VALUE
Parameter: Color
GV_MG_FALLOFF_COLOR
Parameter: Direction
GV_MG_FALLOFF_DIRECTION