How to keep a Garmin GTX330 turned off when powering on avionics?
If pin number 1 on the back of the GTX330 unit is connected to power (+), the unit will turn on. If it is not, you will have to manually press the on button. If you remove power from pin 1, it will not turn it off.
See the installation manual, which is findable by a simple Google/Bing search, page D-1, figure D-1, Note 3.