Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Icon Size
GRAV_ICONSIZEParameter: Object
GRAV_OBJECTParameter: Type
GRAV_TYPEParameter: Strength
GRAV_STRENGTHParameter: Decay
GRAV_DECAYParameter: Mass Dependence
GRAV_MASSDEPParameter: Size Dependence
GRAV_SIZEDEPParameter: On
IN_GRAV_ONParameter: Animation Time
IN_GRAV_ATIME