Rigidbody player movement doesn't response to gravity

Im trying to code a player movement with a use of rigid body.

the movements seems fine but it looks like it doesn't use the default gravity anymore. and when i add force for jumping it doesn't response at all.

Here is the script:

 Vector3 _direction = new Vector3 (Input.GetAxis ("LJoystickHorizontal"), 0, Input.GetAxis ("LJoystickVertical"));
         rigidbody.velocity = _direction * _moveSpeed;
         rigidbody.velocity = transform.TransformDirection (rigidbody.velocity);
         gameObject.transform.position += Vector3.ClampMagnitude(rigidbody.velocity, _moveSpeed) * Time.deltaTime;

What I think is, it is because of the y-axis of _direction is set to 0.

I can't solve this one please help me.