Is modification possible for a leg (stop-loss and target) of the GTT order, in which an entry has been triggered?

Yes, you can modify the price of legs that are triggered and whose status is marked as ‘Open’. However, if the primary leg is triggered and in ‘Open’ state, you can modify the price of target and stop-loss legs too.

In case of a partially executed entry order, the quantity for the exit leg will automatically be updated to the traded quantity on the next trading day.