Software is just a tool same as the camera. What Gareth used to make monsters is probably dated compared to some of the free options today like blender or even the unreal 4 engine which they used for Rogue One.
Best quality I could find but this is the video mentioned for the making of Monsters. Gives some insight into the workflow: https://www.youtube.com/watch?v=z52OHXfLbSU
Great article btw :)