code: public void setGravity(float x, float y, float z) {
          Native3DRigidBody.setGravity(getNative(), x, y, z);
      }
docs: Sets a particular acceleration vector [X, Y, Z] on this {@linkplain GVRRigidBody rigid body}
  
  @param x factor on the 'X' axis.
  @param y factor on the 'Y' axis.
  @param z factor on the 'Z' axis.