What does the z stand for in dragonball z?

It's like the Nissan 350Z. Japanese do that to emphasize something. GT means nothing either, Americans just gave GT a meaning.

* The above answer is not true! *

The Z in Dragonball Z stands for the "Z Fighters".

The Z in Nissan 350Z is a reference to the fact the car is a sports car that can break high speeds.

The GT stands for "Gran Turismo" in Italian and "Grand Touring" in English.