player Gravity haw to Roted?

i have sprite for Gravity who works bat player is not Roted with Gravity if you know that

PlayerController C#

using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour {

	public float moveSpeed;
    public float jumpSpeed = 8.0F;
	public float duckSpeed = 8.0F;
    private Vector3 moveDirection;
   // public float gravity = 20.0F;
    void Update()
    {
        moveDirection = new Vector3(Input.GetAxisRaw("Horizontal"), 0, Input.GetAxisRaw("Vertical")).normalized;

        if (Input.GetButton("Jump"))
        { moveDirection.y = jumpSpeed;   }

		if (Input.GetButton("Duck"))
		{ moveDirection.y = duckSpeed;   }
}
	void FixedUpdate () 
	{
		GetComponent<Rigidbody>().MovePosition(GetComponent<Rigidbody>().position + transform.TransformDirection(moveDirection) * moveSpeed * Time.deltaTime);
	}
}