WJ, gas fired for most gauges is not uncommon in your part of the world (fire safety reasons, mostly). My best suggestion would be for you to join a local Model Engineering Society (Google will help) and ask best advise from there, as they will have experience of these things.
It looks to me to be one of the larger scales like 5" or even 7 1/2", but more than that is difficult from the photos alone. If you use your tape and measure the inside width of the rails it is standing on that will get the correct gauge.
It looks a useful beast and probably able to pull quite a decent train.
Edit: - 7 1/2" might be 7 1/4", though that is more common in the east of USA