how to Apply Gravity at Run time ..?????????

It depends on what are your objects, and what are you trying to do. If the objects are rigidbodies, and you intend to use the standard gravity (Physics.gravity), you can just set rigidbody.useGravity to true/false when needed. On the other hand, if you need a different gravity orientation or value, let useGravity = false and apply a force at FixedUpdate when enabled by the click:

var localGravity: Vector3 = 9.8 * Vector3.up; // inverted gravity var gravityOn: boolean = false;

function OnMouseDown(){ gravityOn = !gravityOn; // toggle the gravity when clicked }

function FixedUpdate(){ if (gravityOn) rigidbody.AddForce(localGravity); }

You need to attach a rigidbody. Make sure it has gravity enabled.