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);
     }
 }