You can often search for a species on the various wood databases online. The databases should tell you about stress resistance, breaking loads, density (i.e. weight) and durability (to rot, termites, boring insects, etc.).
If by "gum" you mean "spotted gum" (apologies if you don't, I am unfamiliar with Australian species) then the wood should be fantastic for building with. It has been used for railway sleepers which indicates that it is both dimensionally stable and durable. The database I have checked also states that it has good resistance to termites, although it is susceptible to boring insects.
Wood database entry for Spotted Gum
Some notes on boring insects, at least in my part of the world,:
They have a strong preference for wet woodThey may attack sapwood for some species but leave heartwoodFinished (waxed, oiled) timbers are unlikely to be attacked
What this means, practically, is that if you can keep your timbers dry and choose heartwood for exposed timbers, you should be OK. Spend time on your roof and drainage around the bottom of the building (most rot happens at the interface between soil and timber as this can be damp and oxygenated) and select your best pieces of wood for the most exposed (or high stress) elements of your building. You can also consider applying linseed/tung oil, or a wax, to exposed timbers (especially end grain which will soak up water like a straw) to increase longevity.
I hope that's helpful! Perhaps someone more local, with experience working with gum, will weigh in and add to or correct me.
