1. GMSprCreateGravityForce (GravityForce.c:43) 
        GMSprForceStruct *GMSprCreateGravityForce(VectorType GVector)
  2. GMSprDestroyGravityForce (GravityForce.c:69) 
        void GMSprDestroyGravityForce(GMSprForceStruct *Force)