This simply means that John Allen didn't run narrow gauge engines - which is quite natural because his G&D RR was a standard gauge line.
Gauge is the distance between the railheads. Standard gauge is 4 ft 8-1/2 " or 1435 millimeters. Most mainline railroads in Northern America and Europe are running standard gauge.
Narrow gauge, logically, has rails which aren't so far apart - but there are lots of variations:
- 3-1/2 ft. (1067 mm) is used a lot in Southern Africa - therefore it is also called Cap gauge.
- Meter gauge - 1000 mm (ca. 3 ft 3") is common in European mountain lines (e.g. in Switzerland)
- 3 ft. (914 mm) is most famous in the Rockies (D&RGW, Rio Grande Southern etc.), but also in the East (East Broad Top RR)
- 2-1/2 ft. (760 mm) or the slightly narrower 750 mm gauge is also used in Europe (Austria has a few famous lines)
- 2 ft. (610 mm): Most famous are the Maine Two-Footers like the Sandy River & Rangeley Lake RR
On the other side there are also wide gauge lines. Examples are 1520 mm in Russia, or even 1668 mm in Spain/Portugal. As far as I know, wide gauge is also common in India (but which one?)...
All in all there are over 130 gauges which were used or still are used today.
For my friends from down under: Sorry, I don't know too much about the gauges used in Australia and New Zealand.

ops: Are you also running Cap gauge or standerd gauge?
Could somebody of you enlighten us?
Ron