At this time propane and gasoline cost approx the same per gallon. As far as any flame effects device goes, you will need to register it, have it inspected and if it passes, you will receive a laminate. The BLM Rangers have been more diligent in the last few years of checking laminates when you are operating a flame effects device or shooting pyro. The laminate is their way of knowing you have resgistered your work and it passed inspection. I have been checked by them on more than one occasion.
This is not something you would be taking from camp to camp. Most of the larger and more powerful devices run on propane. A gasoline powered device would not be permitted to run within the City. It would be an open Playa device only. This is not to say you wouldn't be allowed to bring such a thing. I don't think it would be as reliable or as effective as a propane device unless you created a method to meter the fuel so the fuel/air mixture would be within the correct flammability range. Too little fuel and nothing would happen and too much and the over rich mixture would create a "less than impressive" display. For more information I suggest you contact flame-effects @ burningman dot com. All your questions can be answered at that address.
Those who think they can and those who think they can't are both right.