public GameObject Obj; //prefabを指定しておく
Rigidbody2D Rb2D;

public class PrefabCode : MonoBehaviour {

void Update() {
 if (Input.GetMouseButton(0)) {
  Vector3 position = new Vector3(Random.Range(-10.0f, 10.0f), 10, 0);
  GameObject Prefab = Instantiate(Obj, transform.position, Quaternion.Euler(x, y, z));
  Rb2D = Prefab.GetComponent<Rigidbody2D>();
  Rb2D.gravityScale = 1.0f;
 }
}