Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Source
FIELDLAYER_GRADIENT_SOURCEParameter: Gradient
FIELDLAYER_GRADIENT_GRADIENTParameter: Cycle
FIELDLAYER_GRADIENT_CYCLEParameter: Scale
FIELDLAYER_GRADIENT_SCALEParameter: Offset
FIELDLAYER_GRADIENT_OFFSETParameter: Animation Speed
FIELDLAYER_GRADIENT_ANIMATIONSPEED