I've always found it tough to model gum trees... Mostly due to the lack of suitable materials like tree armatures however with these easy to follow techniques you'll have a paddock full of gum trees in no time!
These trees are a commercially available product from Gaugemaster called Seafoam Trees and with a little bit of pruning you can get this material to look like just about any type of tree you can think of.
In the above video I used seafoam to make gum trees which are found just about everywhere in Australia however they can be found all over the world, additionally all the steps I used to straighten the trunks, paint the branches and apply the foam will apply to all forms of tree you wish to create.
In each package you get enough small branches to make about 35 full trees and enough small offshoots to make nearly 100 small bushes. They are quite expensive to by especially here in Australia however if you live in the US a similar product which is very cheap is called Super Trees from Scenic Express.
If you're in Australia and having a tough time trying to get a similar product you could try Fine Leaf Foliage from Woodland Scenics, it is also the same product as Super Trees however it has been treated and sealed so it meets the requirements for Australian quarantine and can be found at most hobby shops selling Woodland Scenics products.
The only down side to this is any of the natural tree product like Gaugemaster Seafoam and the Woodland Scenics Fine Leaf Foliage needs to be treated before it's permitted in the country thus the price is quite expensive compared to the untreated product from the US hence why it can't be posted to Australia from the US.