Use Addforce() method of a rigidbody compenent, make sure rigidbody is attached to the object and gravity is enabled, something like this
gameObj.rigidbody2D.AddForce(Vector3.up * 10 * Time.deltaTime); or
gameObj.rigidbody2D.AddForce(Vector3.up * 1000);
See which combination and what values matches your requirement and use accordingly. Hope it helps