What is Zerodha GTT triggered but not executed?

A Zerodha GTT order requires a trigger price and a limit price. Sometimes it may happen so that the trigger condition gets hit and the order gets placed to the exchange. However, the order may or may not get executed based on the limit price set. 

A GTT-triggered order does not guarantee execution.

To increase the chances of trade execution, you need to set the buy limit orders above the trigger price and the Sell Limit orders below the trigger price.