Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Icon Size
GRAV_ICONSIZE
Parameter: Object
GRAV_OBJECT
Parameter: Type
GRAV_TYPE
Parameter: Strength
GRAV_STRENGTH
Parameter: Decay
GRAV_DECAY
Parameter: Mass Dependence
GRAV_MASSDEP
Parameter: Size Dependence
GRAV_SIZEDEP
Parameter: On
IN_GRAV_ON
Parameter: Animation Time
IN_GRAV_ATIME