If the gum trees are over 10 feet tall, they are going to be almost impossible to kill with fire. Contracting a forestry mulcher is expensive. I would find someone that does forestry spraying with a skidder spray rig and have them do an application in September. If you get a good kill, you can burn it the following year to clean up the dead trees. After that, a yearly spot spraying can be used to control any new growth.