Define GKEvent

public class Clock : MonoBehaviour
{
    //Just add GKEvent like how you add UnityEvent
    public GKEvent<float> onShowCurrentTime;

    void Update(){
        //Every 1 second, show the current time
        if (Time.time % 1 < 0.01f)
        {
            //Invoke it by using .Invoke
            onShowCurrentTime.Invoke(Time.time);
        }
    }
}