Hello Graeme,
The "Template Preview" option is always disabled by default, in any template and any framework.
Once you enable this Joomla setting, it works just fine with any Gantry 5 template.
Here, have a look at
this video that I recorded for you (watch it in Fullscreen and FullHD for better quality). It shows everything in details.
However, only the
Module Positions will be shown. If you add particles directly in the Layout, they will not be shown.
For more details, please have a look at
this issue on Github. But please take your time and read all comments.