Why is a triggered GTT order that is pending execution not visible?

A triggered GTT order pending execution is not visible because, once triggered, it remains valid for the remainder of the trading day as a CNC (Cash and Carry) limit order with Day validity. At the end of the day, the exchange treats it like any other normal day order and cancels it. As a result, the order is not visible from the next day onwards. To learn more about validity, see What does Day, Immediate and Minutes Validity mean?