Gravity To CharacterController

Hi, I'm have issues adding gravity to the code below. Can someone please assist with this.

 private var speed = 2.0;
 private var pos : Vector3;
 private var tr : Transform;
 
  
 function Start() {
     pos = transform.position;
     tr = transform;
 }
  
 function Update() {
  
     if (Input.GetKey(KeyCode.D) && tr.position == pos) {
         pos += Vector3.right;
     }
     else if (Input.GetKey(KeyCode.A) && tr.position == pos) {
         pos += Vector3.left;
     }
     else if (Input.GetKey(KeyCode.W) && tr.position == pos) {
         pos += Vector3.forward;
     }
     else if (Input.GetKey(KeyCode.S) && tr.position == pos) {
         pos -= Vector3.forward;
     }
      
     transform.position = Vector3.MoveTowards(transform.position, pos, Time.deltaTime * speed);
    
   
    
 
 
 }