My first question is basic and I hope that it doesn't insult you, but make sure that the voltage on the ballast and the voltage on the HPS are the same. Did the electrician run a 240 or a 120 circuit? Is your ballast set up for the same voltage? Like the others mentioned, I think I would try a toaster or a blow dryer in the outlet and see if everything trips it. How many dedicated circuits did you run? Have you tried other outlets? You probably only need 1 dedicated 20 amp circuit for an entire 1000W grow space.
I'm not an electrician, but I am not putting a lot of confidence in what the HTG guy said. I have a GFI in my grow room and a 1000W HPS. I personally would not have a grow room that was not GFI protected, we are dealing with such high wattage. While it is not the outlet that I plug my light into, it is the first circuit in the room so protects all the outlets that are run off the same breaker. When I ran the dedicated circuit, we put in several outlet boxes around the perimeter of the space that run on the same breaker. I have a timer plugged into the second box on the circuit for the 1000W. Last winter, I also ran a 600W light off the same circuit.
Also, the fact that it is tripping the breaker and not tripping the GFI tells me that the GFI is probably not the problem.