Itween follow path add gravity to game object following the path?

Hello i have a script that makes my player move and change paths but how can i have him be effected by gravity and still stay on the path?
here is my code:

using UnityEngine;
using System.Collections;

public class follow : MonoBehaviour {
    private float smoothing = 1.0f;
    private Vector3 targetPosition;

    public float runSpeed = 0;

    public Transform[] path0;
    public Transform[] path1;
    public Transform[] path2;

    private Transform[][] paths = new Transform[3][];
    private int iPath = 0;


    float percentsPerSecond = 0.02f; // %2 of the path moved per second
    float currentPathPercent = 0.0f; //min 0, max 1

    void Start () {
       paths[0] = path0;
       paths[1] = path1;
       paths[2] = path2;
    }

    void Update () {                   
           if (Input.GetKeyDown("a")) {
                if (iPath == 2) iPath = 1; else iPath = 0;

           }

           else if (Input.GetKeyDown("d")) {
                if (iPath == 0) iPath = 1; else iPath = 2;
           }
       currentPathPercent += percentsPerSecond * Time.deltaTime;
       iTween.PutOnPath(gameObject, paths[iPath], currentPathPercent);
       transform.LookAt(iTween.PointOnPath(paths[iPath],currentPathPercent+.05f));
    }

    void OnDrawGizmos(){
       iTween.DrawPath(path0);
       iTween.DrawPath(path1);
       iTween.DrawPath(path2);
    }
}

sorry solved