Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Source
FIELDLAYER_GRADIENT_SOURCE
Parameter: Gradient
FIELDLAYER_GRADIENT_GRADIENT
Parameter: Cycle
FIELDLAYER_GRADIENT_CYCLE
Parameter: Scale
FIELDLAYER_GRADIENT_SCALE
Parameter: Offset
FIELDLAYER_GRADIENT_OFFSET
Parameter: Animation Speed
FIELDLAYER_GRADIENT_ANIMATIONSPEED