How do I add gravity to an object?

Hi there, I have got my hands on a script which allows me to pick up objects and move them...

 private var pickObj: Transform = null;
 private var hit: RaycastHit;
 private var dist: float;
 private var newPos: Vector3;
 
 function Update(){
 
     if (Input.GetMouseButton(0)){ // if left button creates a ray from the mouse
         var ray = Camera.main.ScreenPointToRay(Input.mousePosition);
         if (!pickObj){ // if nothing picked yet...
             if (Physics.Raycast(ray, hit) && hit.transform.tag == "Pick"){
                 // if it's a rigidbody, zero its physics velocity
                 if (hit.rigidbody) hit.rigidbody.velocity = Vector3.zero;
                 pickObj = hit.transform; // now there's an object picked
                 // remember its distance from the camera
                 dist = Vector3.Distance(pickObj.position, Camera.main.transform.position);
             }
         }
         else { // if object already picked...
             newPos = ray.GetPoint(dist); // transport the object
             pickObj.position = newPos;   // to the mouse position 
         }    
     }
     else { // when button released free pickObj
         pickObj = null;
     }
 }

But I want to add gravity to the objects so that when I let go or press the button again the object drops to the floor / Ground!? Thanks -Izzy